The model protein antigen, human gamma globulin (HGG) was microencapsulated in poly(DL-lactide-co-glycolide) microparticles and administered orally to rainbow trout. Oncorhynchus mykiss (Walbaum, 1792). Using a Western blotting technique it was demonstrated that the dynamics of passage through the gut were different for the microencapsulated and soluble antigen. The association of HGG with microparticles increased the retention time of the antigen in the stomach and delayed its entry into the intestinal region. After the delivery of microencapsulated HGG, antigen was detected in gut contents in fragmented form which suggested that some of the antigen was present at the particle surface and therefore susceptible to proteolysis. However, a greater amount of intact antigen was detected in the posterior intestine and in the bloodstream of fish, which were administered with microparticle-associated than soluble antigen, indicating that the antigen was partially protected. Immunization with microencapsulated HGG resulted in the detection of specific antibody in the serum but levels were not significantly greater than after the delivery of soluble antigen. However, specific antibody was detected in the intestinal mucus of fish which were administered with the microencapsulated antigen after boosting with soluble HGG but not in fish which were primed with the soluble antigen.