A base population (n = 101) of carp, consisting of a single hybrid cross, was immunized with the hapten-carrier complex DNP-KLH, to perform a divergent selection for antibody response. Measurement of the DNP-specific antibody response at 12 and 21 days postimmunization, allowed the classification of a low number of individual carp as early/high (10%) or late/low (13%) responders. Three individuals defined as early/high and three defined as late/low responding, were gynogenetically reproduced to obtain corresponding homozygous progenies within one generation only. Upon immunization with DNP-KLH, the antibody response was found to be significantly higher in the early/high responder homozygous offspring. Although the homozygosity of the offspring apparently caused a (s)lower antibody response (compared with the base population), the differences between the high and low responder offspring to indicate a genetic influence on the antibody response. The realized heritability (h2) for antibody production was estimated at 0.37 +/- 0.36. The present study provides the basis for a divergent selection of homozygous inbred carp lines with a genetically controlled difference in antibody response. These inbred lines will allow us to investigate relationship(s) between immune responsiveness and resistance to infectious diseases in fish.