The primary and the secondary immune responses of rainbow trout, Salmo gairdneri, and white rats, Rattus norvegicus, (Wistar strain) to keyhole limpet haemocyanin (KLH) and Escherichia coli lipopolysaccharide (LPS) have been investigated. The humoral immune responses were detected by enzyme-linked immunosorbent assay (ELISA) using urease as the conjugated enzyme. The number and time of appearance of stimulated lymphocytes were investigated by the detection of antibody-secreting cells (ASC) and antigen-binding cells (ABC) in the spleens of both species and the anterior kidneys of rainbow trout. In fish, both the humoral and lymphocyte secondary responses were significantly higher than primary responses when LPS was used as antigen but not in the case of KLH. With rats the secondary responses were significantly higher for both antigens. The two antigens were introduced into members of both species either simultaneously or sequentially. Generally, LPS had a synergistic effect on the response to KLH when it was injected at the same time. The effect was variable when the antigens were given at different times. Antigen localisation in the lymphoid tissues of both animals was investigated by the injection of live E. coli. Two techniques were used: colony counts after growth on solid agar and peroxidase anti-peroxidase (PAP) localisation of the bacteria within the tissues. The first technique was used to detect the number of live bacteria in the blood, liver, spleen and muscle of injected rats and fish as well as in the anterior kidneys of fish and the lymph nodes of rats. Generally, the spleen, liver and blood in both species and the anterior kidney in fish and the lymph node in rats were the main organs in which live bacteria were found. The rate of clearance of live bacteria was variable between and within fish and rats. The PAP technique detected the existence of both live and dead bacteria. In fish, the bacteria were localised in a diffuse pattern during the first few days after injection, but later they tended to localise in the ellipsoids of the spleens and around the melano-macrophage centres in spleens and anterior kidneys. In rats the localisation of the bacteria was less diffuse and it was concentrated mostly in the white pulp of the spleens.