An immunohistochemical method using formalin-fixed, paraffin wax-embedded sections is described for detecting strain-specific major histocompatibility complex class I antigens in knee-joint tissue from DA and Lewis strains of rat. The fixed osteochondral tissues were additionally decalcified in formic acid before processing for paraffin wax embedding. For immunohistochemistry, two monoclonal antibodies, one specific for DA class I allele RT1Aa and the other for Lewis class I allele RT1A1, were used together with the avidin-biotin immunoperoxidase procedure. It was necessary to use strain-specific normal rat serum as a diluent for the antibodies to suppress cross-strain recognition. DA-specific antibody stained positively only on DA rat sections, not on Lewis rat sections, and Lewis-specific antibody stained positively only on Lewis rat sections, and not on DA. Positive staining was localized in the bone marrow, osteochondral cells and endothelium. We propose that the use of a decalcification medium may have enhanced the immunoreactivity of the tissue. The method described can be used on sections of allografts from the two strains of rat to assess morphologically the extent of cellular replacement of the graft by the host's cells.