Monoclonal antibodies (MoAb) that cross-react with the shared epitopes of S100 protein have been prepared from mouse hybridoma cell lines and partially characterized. Nine of these MoAb were applied to sections of formaldehyde-fixed paraffin-embedded human tissues that were stained by immunohistochemical techniques. Three of these MoAb give uniformly and reproducibly positive staining in appropriate cell types when stained by avidin-biotin methods. Three of the MoAb were judged to be negative, although some MoAb gave inappropriate staining patterns. The three remaining MoAb showed either great heterogeneity in their staining patterns or intensities, or gave a lesser degree of reproducibility in a given tissue or neoplasm. One of the MoAb designated 15E2E2 that belonged to the first group of reproducibly staining antibodies was used to stain a larger number of normal human tissues and neoplasms. The staining that was observed appeared to recapitulate that which was previously described for conventional S100 protein antibodies. Monoclonal antibodies may, therefore, have a role in selected cases where standard microscopy is equivocal for a specific tissue diagnosis, or where independent verification of the diagnosis would be beneficial.