First Minister Rhodri Morgan has promised that the official inquiry into the south Wales E.coli outbreak will be independent and open.
But he strongly rejected as "ridiculous" any suggestion his government's response had been slow.
A week after the outbreak emerged, and as cases rose once more, to 115, Mr Morgan said lessons had to be learned.
Opposition parties had demanded that the inquiry must be fully public and said it should have been called sooner.
Health Minister Brian Gibbons announced on Friday that there would be an assembly government inquiry into Wales' worst E.coli outbreak, which has mostly affected children, at 32 schools in the valleys.
Twenty-five people have been taken to hospital so far, with four children still receiving treatment.
It has been linked to a Bridgend meat supplier, John Tudor and Son, although officials have said there is no conclusive evidence so far.
The first seven cases were confirmed last Sunday, and the total increased steadily throughout the week, topping 100 on Saturday.
Mr Morgan told BBC Radio Wales' Sunday Edition he was "absolutely" committed to a full inquiry.
"There is no difficulty about having to make this as open and as independent and as transparent as possible," he said.
"I can't give you the details of the inquiry now, but simply a commitment that it will be an open inquiry, because the lessons must be learned."
Conservative AM Jonathan Morgan AM has said the inquiry should have been announced earlier when the apparent source of the E.coli was identified, and when it was clear cases would rise.
But asked whether the response had been slow, Mr Morgan said: "No, no, don't start pointing the finger of blame.
"People involved in the outbreak control team... have been working absolutely flat out and keeping Brian Gibbons and myself notified.
"They are working from seven in the morning until 11 at night. I will not have this criticism - it is ridiculous."
Plaid Cymru AM Leanne Wood has questioned whether the contracting out of school meals should end and called for better inspection of food suppliers.
She also suggested Wales should follow Scotland after its fatal E.coli outbreak in 1996 and set up a task force to protect people.