By KAREN MATTHEWS
Associated Press Writer

NEW YORK - Tests for anthrax at ABC, where a 7-month-old boy was believed to have contracted the disease, and other media organizations in New York have come back negative, the mayor said Sunday.

The baby, the son of an ABC producer, had visited the network's offices, leading investigators to believe the infection may have occurred there. Mayor Rudolph Giuliani said investigators had not identified the source of the infant's infection, and no suspect letter had been found.

Giuliani said the number of New Yorkers infected with the bacterium remained at four, with one case each at NBC, CBS, ABC and the New York Post.

Environmental samples from several other media outlets and schools had all yielded negative results, Giuliani said. Several results were still pending. He did not list the organizations that were tested.

Most of the tests were environmental, but at NBC more than 1,300 people have been tested for exposure to anthrax. All came back negative, the mayor said.

An anthrax-contaminated letter was found unopened Friday in the mailroom of the New York Post, where an employee was diagnosed with anthrax last week. It was addressed to "Editor, New York Post" in similar block letters to those sent to NBC anchor Tom Brokaw and Senate Majority Leader Tom Daschle.

It remained unclear Sunday what infected the employee, Johanna Huden, an assistant to the newspaper's editorial page editor. Huden said she remembers opening another piece of mail addressed in block capital letters, the newspaper reported.