Abstract In carps ( Cyprinus carpio L.) and tortoises ( Testudo hermani GMELIN and Agrionemys [ Testudo] horsfieldii GRAY) it was possible to detect antibodies against the DNP group after immunization with a DNP-protein conjugate. By passive hemagglutination tests the serum titers of antibodies against the DNP group and against the carrier protein used for immunization were studied during the immune response in tortoises and found to be nearly parallel. The maximum of the precipitating anti-DNP antibodies compared with the antibodies detected by passive hemagglutination appeared in a later stage of the secondary immune response. In comparison with anti-DNP antibodies from rabbits the point of maximal precipitation of the antibodies of these lower vertebrates is shifted to higher antigen concentrations. For isolated carp immunoglobulins with antibody activity a sedimentation constant S c 20°C of 13S was determined.