Mr Mignini is angry at what he perceives as interference in the trial
The prosecutor in the Meredith Kercher case has hit back at American supporters of accused student Amanda Knox who say he is "mentally unstable".
Giuliano Mignini told the BBC he had "never visited a psychologist" and he was taking legal action against a US paper that carried the allegations.
Mr Mignini also said Ms Knox's backers were trying to "influence" the trial.
Amanda Knox and her Italian boyfriend are accused of the murder of British student Meredith Kercher in 2007.
Ms Knox and Raffaele Sollecito will appear in court again on Friday when four British friends of Meredith Kercher are expected to take the stand.
Mr Mignini said he was "not happy" about a story on the West Seattle Herald's website last month in which supporters of Amanda Knox say he is believed to be mentally unstable.
Speaking to the BBC from his office in Perugia, he stressed he was speaking in a personal capacity, and not as the official prosecutor in the Kercher trial.
The Herald, which is based in Amanda Knox's home town, had been covering a "Friends of Amanda" event to raise money for the accused student.
The report from the event quoted unnamed supporters as alleging that Mignini is "using the high-profile Amanda Knox case to improve his dicey reputation and further his career".
It continued: "Some in both the American and Italian legal field believe Mignini is mentally unstable."
No-one at the West Seattle Herald could be reached for comment.
Mr Mignini confirmed he has started an action for defamation against the newspaper.
He joked: "I am quite a healthy man. I don't go to the doctor much and I have never visited a psychologist."
The allegations are the latest episode in what Mr Mignini believes to be a systematic attempt to discredit him, and thus derail Amanda Knox's trial.
He said: "These are allegations from 9,000 kilometres away from people who have no knowledge of me and to whom I have never spoken.
"I would never give an opinion on someone I know nothing about.
"I regard it as trying to influence the trial. These things might happen in Italy but I really would not expect attempts to influence to come out of the United States."
Amanda Knox's supporters have been consistently critical of Mr Mignini's past record, and the way in which he has conducted the prosecution case in the Meredith Kercher trial.
They dispute the prosecution case - that Amanda, Raffaele and a third man, Rudy Guede, murdered Meredith during a "sex game" in Meredith's Perugia bedroom.
Instead they maintain that Guede - who was last year found guilty and sentenced to 30 years imprisonment for Meredith's murder - acted alone.
The supporters say that, contrary to some press reports, Amanda Knox never confessed to the crime, and that the DNA and other evidence linking her to the crime is flawed.
The case against Amanda Knox is flawed, say supporters
Evidence that the trial's prosecutor is also being targeted by Ms Knox's supporters appears prominently on the website of Seattle lawyer Anne Bremner, who represents the Friends of Amanda.
They include accusations that he leaked "false information" to the press and that Mr Mignini is under indictment for "abuse of office".
The indictment allegation is understood to refer to a previous case that Mr Mignini investigated in Florence.
But Mr Mignini said it was true that although a Florence prosecutor had brought proceedings against him, another court had already "declared non-existent" the charges of abuse of office.
Mignini is also quoted as being "in thrall to a sort of delirium" in his handling of the Florence case, in which he "fantasized amazing and complex Satanic conspiracies."
This is believed to be a reference to Mr Mignini's involvement in an inquiry connected to the infamous "Monster of Florence" serial killings, during which Mr Mignini is said to have consulted an alleged psychic, Gabriella Carlizzi.
Said Ms Bremner: "We have concerns that the prosecutor has in the past relied upon a medium."
But Mr Mignini said he was "not friendly" with Mrs Carlizzi, and did not share her views, even to the point of having her arrested in 2005.
"I have said these things many times to American journalists," he said. "But there are none so deaf as those who will not hear."
Ms Bremner told the BBC that the Friends of Amanda in no way endorsed the allegations of mental instability against Mr Mignini.
"We have not made any personal attacks," she said.
"Everything we do is truth-based and not negative."
Additional reporting by Danielle Gambazza