Some properties of mouse antibodies produced by hyperimmunization with conjugated haptens, emulsified in complete adjuvant, have been investigated. Under these conditions two antigenically different types of precipitating antibody, with small differences in their electrophoretic mobilities, both migrating in the gamma-region, are produced. Both antibody types were shown to be different from the beta(2A)-globulins) as revealed by the absence of reaction with specific rabbit antisera against mouse beta(2A)-myeloma globulins. gamma(2) or the slower migrating antibodies, similarly to what has been described for the guinea pig, were able to provoke lysis of antigen-coated tanned sheep erythrocytes, in the presence of complement. This activity could not be detected in the faster migrating antibody type. The slower migrating fractions of the gamma(2)-antibodies were able to transfer passive cutaneous anaphylaxis (PCA) in the guinea pig, but not in their own species. gamma(1)-Antibodies were found to be capable of sensitizing the mouse for cutaneous anaphylaxis. The sensitivity of these antibodies to reduction and alkylation was different, the lytic activity being practically abolished, the PCA titers in mice much reduced, and the PCA activity in guinea pigs only partially inactivated.