Major histocompatibility (MH) class II receptors are expressed on the surface of specialized antigen-presenting cells in vertebrate immune systems. Their function is to present peptides derived from exogenous pathogens to CD4+ T cells. Variation in the level of expression of these genes has been linked to pathogenesis in various diseases. Very little has been published on the function of MH class II receptors in teleost fish to date. In this study, we have produced polyclonal antibodies recognizing MH class II alpha and beta proteins of rainbow trout and employed them to characterize the expression pattern of these genes. Deglycosylation using N-glycosidase F and endoglycosidase H showed that MH class II alpha is glycosylated in rainbow trout. MH class II beta was also found to be glycosylated as reported previously. Results from Northern blotting revealed that the expression of these genes was not affected by exposure of rainbow trout to temperature of 5 degrees C. However, at 2 degrees C, downregulation of MH class II alpha and beta genes was evident at both the mRNA and protein levels as assessed by Northern and Western blotting, respectively. Because MH class II antigens play an important role in generating an immune response to bacterial and fungal pathogens, downregulation of these genes at low temperature could account for the susceptibility of fish to low temperature-related diseases such as bacterial cold-water disease and winter saprolegniosis.