A group of 269 pupils of the Harbour and Transport Training Institute in Rotterdam (group A), aged 13-20 years, and of 109 patients of the Dr Mr Willem van den Bergh Foundation at Noordwijk (group B), aged 11-21 years, were immunized with a whole virus vaccine containing 10, 20, or 40 microgram HA of A/USSR/92/77 (H1N1) influenza virus. A booster vaccination was administered 6 weeks later with 20 microgram HA of the same virus. Many of the participants had been immunized during the two preceding years with a whole virus vaccine containing A/New Jersey/8/76 (H1N1) (A/NJ/76) virus. The side-effects, mostly of a moderate nature, increased with the dose of virus in the vaccine. In group A side effects were least frequent in the vaccinees who had never received A/NJ/76 vaccine. A single dose of A/USSR/77 vaccine did not produce satisfactory levels of homologous antibodies. After booster immunization with 20 microgram HA of A/USSR/77 virus participants showed a higher homologous antibody response in all vaccine-dose groups if they had not been immunized with A/NJ/76 virus in previous years. After primary and especially after booster immunization with A/USSR/77 virus, a very high response against A/NJ/76 virus and adequate levels of A/NJ/76 antibody were found in participants who had been immunized previously with A/NJ/76 virus. Those who had not been immunized with this virus previously showed no or a very low antibody response to A/NJ/76 virus.