A field evaluation of two formalininactivated respiratory virus vaccines was conducted in a selected pediatric population in California during the 1966—1967 respiratory disease season. A total of 441 children ranging in age from four months to nine years were immunized; 219 with a respiratory syncytial (RS) virus vaccine and 222 with a trivalent parainfluenza virus (types 1, 2 and 3) vaccine. Both vaccines elicited good serum antibody responses. Very high attack rates of parainfluenza virus, types 1 and 3 and RS virus were observed during the study period in infants and children in both vaccine groups. A protective effect was not demonstrable for either vaccine. Infants who received the RS virus vaccine and who subsequently became infected with RS virus tended to have a more severe clinical illness than infants who did not receive this vaccine.