The mAb HCA2 reacts preferentially with HLA-A locus heavy chains. Its reactivity contrasts with that of HC10, a mAb with more pronounced specificity for HLA-B and -C heavy chains. Both HCA2 and HC10 were raised against free class I heavy chains of HLA-A and -B antigens respectively, to obtain mAbs that would still react with denatured class I antigens, as they occur in Western blotting, conventional light microscopical analysis of formalin-fixed, paraffin-embedded sections, and cryo-immuno-electron microscopy. HCA2 and HC10 indeed retain strong reactivity with free class I heavy chains in Western blots. The HCA2 mAb in particular reacts in a locus-specific manner by biochemical criteria. Conditions are described for use of HCA2 and HC10 in immunohistochemical staining of formalin-fixed, paraffin-embedded sections. HCA2 and HC10 also produce strong reactivity in immuno-electron microscopy. Their use allows the determination of tissue and subcellular distribution of class I antigens.