1. A virus capable of producing pulmonary consolidation in the wild mongoose (Herpestes griseus) has been isolated from throat washings obtained from four patients with a clinical syndrome termed acute pneumonitis.
2. The virus was not pathogenic for ferrets, mice, guinea pigs, rabbits, monkeys, voles, hamsters, deer mice, skunks, opossums, or woodchucks.
3. The virus was filterable through Berkefeld V and N candles, was not inactivated by glycerin or by freezing and drying in vacuum, and was propagated for at least 30 serial passages on the chorio-allantoic membrane of the developing chick embryo.
4. Normal mongooses placed in contact with infected mongooses developed pulmonary consolidation.
5. The virus was neutralized by the serum of mongooses convalescent from the infection but was not neutralized by normal mongoose serum.
6. Serum of human beings convalescent from acute pneumonitis also neutralized the virus, but serum obtained from the same individuals during the acute phase of the disease failed to do so.
7. The evidence so far obtained strongly suggests that this virus is the cause of acute pneumonitis in human beings. It differs from other viruses known to cause infections of the respiratory tract in man.