The anti-haemagglutinin antibody response in adult human volunteers to inactivated whole virus or tween ether split influenza A/Victoria/75 (H3N2) and A/Scotland/74 (H3N2) virus vaccines was investigated using antibody absorption and single-radial-haemolysis (SRH) techniques. The concentrations of haemagglutinin (HA), nucleoprotein (NP) and matrix (M) antigens measured by single radial diffusion (SRD) and rocket immunoelectrophoresis were similar for both the whole virus and split vaccines. Whole virus and split vaccines induced crossreactive (CR) antibody in 87% of vaccinees. Strain specific (SS) antibody to A/Hong Kong/1/68 of the homologous virus was induced less frequently than CR antibody. Higher anti-haemagglutinin antibody titres were detected in persons receiving the split virus vaccines than in those receiving the whole virus vaccines. No antibody to the type-specific matrix protein was detectable, but 33% of volunteers developed an antibody rise to type-specific nucleoprotein antigen. The specificity of the anti-haemagglutinin antibody response in human adults to natural infection with A/Port Chalmers/73 (H3N2) virus was similar to that induced by inactivated vaccines in that a high proportion of subjects developed CR anti-haemagglutinin antibody, which reacted with A/Hong Kong/68 virus and the homologous A/Port Chalmers/73 virus, and SS antibody for A/Hong Kong/68 virus but SS antibody for A/Port Chalmers/73 virus was infrequently stimulated by natural infection.