The immunogenicity and antigenicity of human coronaviruses 229E and OC43 were studied by quantitative immunoelectrophoresis. Three distinct antigens were recognized in both coronavirus strains when T-100-solubilized whole virus was tested by two-dimensional immunoelectrophoresis against homologous rabbit antiserum. No antigens cross-reacted between strains, but the electrophoretic patterns against homologous antiserum were highly similar in that both strains had one electrophoretically fast antigen, one of intermediate mobility, and one of slow mobility. Immunization of rabbits with 10(9) plaque-forming units of virus was required for production of antiserum which recognized the three antigens; lesser amounts gave rise to antisera which recognized only one or two components. Precipitin lines excised from two-dimensional immunoelectropherograms were used successfully as immunogens to prepare monospecific antiserum to each of the antigens of OC43 and 229E. Monospecific antiserum to the slow component of 229E neutralized 229E only, and monospecific antiserum to the slow component of OC43 both neutralized and inhibited hemagglutination of OC43 virus. Human convalescent sera which possessed both complement-fixing and neutralizing antibody also recognized the slow-moving component.