Three hundred volunteers were divided into two age groups, 14-30 years and 31-60 years. Each participant was immunized intramuscularly with a subunit, whole virus or absorbed whole virus vaccine, containing A/Bangkok/1/79 (H3N2), A/Brazil/11/78 (H1N1) and B/Singapore/222/79 influenza virus. Serum haemagglutination-inhibition (HI) antibody response, protection, and reactogenicity were studied after one and two doses of the vaccines. Primary immunization induced much higher percentages of HI antibody titres greater than or equal to 100 against all three vaccine viruses and much higher geometric mean titres (GMT) in volunteers with pre-immunization titres greater than or equal to 18 as compared to those with pre-immunization titres less than 18. Secondary immunization did not result in an increase of GMTs or antibody titres greater than or equal to 100 in volunteers with pre-immunization titres less than 18. On the whole, the response to the subunit vaccine was similar to that to the other two vaccines. To influenza B/Singapore/222/79 virus the response was lowest after administration of the whole virus vaccine in the age group 31-60 years. Over 50% of the HI titres greater than or equal to 100 found after immunization in the different vaccine and age groups were still present after one year. Serologically established infections during the winter months following immunization amounted to 15% in the subunit vaccine group, 6% in the whole virus vaccine group, and 10% in the adsorbed whole virus vaccine group. Local and systemic reactions to all three vaccines were mild in nature. Local reactions after primary immunization were much less frequent following administration of the subunit vaccine as compared to the other two vaccines, especially in the younger age group. In comparison to primary immunization, after booster immunization the incidence of local reactions was higher for the subunit vaccine and lower for the adsorbed whole virus vaccine. In the age group 14-30 years the incidence of local reactions after primary as well as booster immunization was much greater in females than in males, especially when the adsorbed whole virus vaccine was used.