Significant increases in neutralizing antibodies were demonstrated in 42 of a total of 69 persons with a clinical diagnosis of primary atypical pneumonia. Detailed titrations of virus-neutralizing antibodies in a representative group of 28 patients are presented. Increases of four- to 64-fold were demonstrated. Acute-phase titers were 4 or less in 83 per cent and convalescent titers were 16 or over in 86 per cent of these cases.

Only about half of the number of patients having increases in neutralizing antibodies also developed cold agglutinins and agglutinins for the indifferent streptococcus No. 344.

Patients from the Eastern United States as well as those from the Pacific Coast were shown to develop virus-neutralizing antibodies.

Patients with pneumococcal pneumonia and pneumonias caused by influenza virus type A or viruses of the psittacosis group did not have significant increases in neutralizing antibodies for the virus of atypical pneumonia. Cold agglutinins appeared in 3 cases of type A influenzal pneumonia.

Sera from persons with atypical pneumonia, when tested against the 3 most prevalent respiratory viruses isolated from cotton rats and hamsters, failed to neutralize these agents or showed no significant change in neutralization titer.

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