Rainbow trout (Salmo gairdneri) were immunized by flush exposure or intraperitoneal injection with single doses of the following dinitrophenyl (DNP) conjugates: DNP-keyhole limpet hemocyanin (KLH), DNP-Ficoll, DNP-O-antigen (from the fish pathogen, Yersinia ruckeri), DNP-sheep red blood cells, or DNP-duck red blood cells. The immune response was demonstrated by the passive hemolytic plaque assay to show splenic antibody-producing cells (APC) 14 days after antigen administration. High numbers of splenic APC specific for the hapten were found when the conjugates DNP-KLH, DNP-Ficoll, and DNP-O-antigen were given by injection and when DNP-O-antigen was given by flush exposure. The heterologous red blood cells were relatively nonimmunogenic. When the immune response to the hapten and carrier could be measured--e.g., after immunization with the DNP-O-antigen--the numbers of APC were consistently higher for the hapten.