Endoplasmic reticulum (ER) chaperones of the antigen processing machinery play a crucial role in HLA class I antigen complex assembly and antigen presentation. The characterization of the expression of these chaperones in normal tissues and malignant lesions has been hampered by the lack or limited availability of ER chaperone-specific monoclonal antibodies (mAb) that are suitable for immunohistochemical staining. To overcome this limitation, we have generated human calnexin, ERp57, calreticulin and tapasin-specific mAb-secreting hybridomas from BALB/c mice immunized with peptides and recombinant proteins. The mAb TO-5, TO-2, TO-11 and TO-3 were shown to be specific for calnexin, ERp57, calreticulin and tapasin, respectively, as they react specifically with the corresponding immunizing peptides in ELISA and with the corresponding proteins when tested with human lymphoid cell lysates in Western blotting. Furthermore, the reactivity of the four mAb with the corresponding intracellular antigens yielded intracellular staining when the mAb were tested with formalin-fixed, microwave-treated and saponin-permeabilized cells in indirect immunofluorescence and with formalin-fixed, paraffin-embedded tissue sections in the immunoperoxidase reaction. These results suggest that the ER chaperone-specific mAb we have developed are useful probes for characterizing the expression of ER chaperones of the antigen processing machinery in normal and pathological cells. This information will contribute to defining the effects of abnormalities in their expression on HLA class I antigen expression and function and on the interactions of target cells with the host's immune system.