The reactogenicity and immunogenicity of whole-virus and split-product influenza vaccines were studied in 77 children between the ages of 6-36 mo. Subjects initially received monovalent vaccine containg either A/USSR/77 (H1N1) antigen in 1978 or A/Brazil/78 (H1N1) antigen in 1979. One mo. later, a trivalent preparation was given which contained the respective H1N1 antigen plus A/Texas/77 (H3N2) and B/Hong Kong/72 antigens. Temperatures of .gtoreq. 37.8.degree. C (.gtoreq. 100.degree. F) were observed more commonly after initial vaccination with whole-virus vaccine (35%) than after split-product vaccine (14%). No child had a temperature of .gtoreq. 39.4.degree. C (103.degree. F) or a febrile convulsion. The trivalent vaccines were more reactogenic than the monovalent vaccines although none of the reaction indices exceeded 0.9. The whole-virus vaccine appeared to be more immunogenic especially in those children who were initially seronegative (preimmunization hemagglutination-inhibiting antibody titer < 5). Only 50% of children vaccinated with split-product vaccines with initial hemagglutination-inhibiting titers of < 5 achieved titers of .gtoreq. 20 to the H1N1 antigen after 2 doses of vaccine compared with with 97% in similar whole-virus vaccine recipients. The degree of antibody response to the A/Texas/77 component of the vaccines was greater than the response to the A/Brazil/78 or A/USSR/77 antigens.