Three institutions were targeted with suspect mail
None of the Jang newspaper's staff in Karachi were found to have been exposed to the bacteria.
Pakistan's leading hospital, which confirmed the letter as testing positive for anthrax, said that the letter will be sent abroad for further tests.
The Urdu-language newspaper was one of three institutions sent letters containing white powder; a bank and a computer business were also targeted.
The newsroom was sealed and workers in protective gear sprayed the area as staff were treated with antibiotics as a precaution.
Motive unclear
The newspaper's editor, Mehmood Sham, told the BBC he had no idea why his publication had been targeted.
He said that although the paper supported the government's stand against terrorism it had also given "vast coverage" to parties opposing the government's position.
The Pakistani Government acted quickly to calm public fears, saying it had set up a special cell in the Interior Ministry to deal with any potential threat.
It said it had enough antibiotics to cope with an outbreak.
The country's Science Minister, Atta-ur-Rehman, said people had as much chance of contracting anthrax as being run over by a car or struck by lightning.
He added that Pakistan did not have the facilities to produce the type of anthrax found in the letter, which purported to include a press release from a social welfare organisation.
Hoax letters
Daily Jang offices in two other cities, Quetta and Rawalpindi, also received letters claiming to contain anthrax spores, although no infection has been reported there.
The Daily Jang's English-language sister publication, The News, said the letter sent to the Quetta office contained a short message: "Anthrax gift for you and your staff".
There have now been several anthrax scares in Pakistan, including alerts at the US embassy and the British High Commission in Islamabad.
The US embassy said two weeks ago that white powder in a letter it received had tested negative for the disease, Reuters news agency reported.