The National Institute of Health is now having second thoughts about a man whom it announced as the nation's first "probable" case of SARS.

The agency says it's highly likely that the man in his 40s may be suffering from a bacterial pneumonia ... and not the viral one which is linked to the deadly illness.

The agency said the patient responded well to an antibody fighting bacterial pneumonia ... with body temperature dropping under 37 degrees, and the white blood cell count falling from 20,000 to nine 9,000. Chest X-rays also showed a considerable drop in pneumonia symptoms.

SARS is believed to be caused by a family of the coronavirus.

The institute plans to make a final determination on the case in two days after further tests of sample specimens taken from the patient.

On Tuesday, the National Institute of Health said that the man, who returned from Beijing Monday, may be the first SARS patient in Korea.