In a previous paper from this laboratory, the production of monoclonal antibodies recognizing antigenic determinants common to the alpha and beta chains of bovine brain S-100 protein was reported. In the present study, the immunohistochemical labeling patterns of these monoclonal antibodies against a wide range of normal and pathologic human tissues are described, and these results are compared with those obtained using polyclonal anti-S-100 antiserum. Although many of the reactions of the monoclonal antibodies were very similar to those of the polyclonal antiserum (and the previously reported sites of S-100 immunoreactivity) several additional cell types (e.g., thyroid follicular cells, biliary epithelium, pancreatic cells, renal tubules) were labeled by one or both of the monoclonal antibodies. Blocking experiments prove that immunohistochemical differences obtained with monoclonal and polyclonal antibodies are at least partly caused by differences in the repertoire of antigenic determinants on S-100 protein that are recognized by the two species (i.e., mouse and rabbit, respectively). These results indicate that S-100 may be more widespread in human tissues than previously thought, and that its value as a marker of the histogenetic origin of human tumors should be reappraised. It is suggested that the marked variability observed in the reactivity of different tissues in the present study may indicate that S-100 is a heterogeneous group of molecules, and its expression may be related to the functional activity of cells.