RAP-5 is a monoclonal antibody that has been shown to be immunoreactive with human ras gene product p21 in solid-phase radioimmunoassays and Western blots. RAP-5 binding in excess of that of control monoclonal antibodies to formalin-fixed tissue sections of several types of human tumors has been reported, and this binding has been interpreted as indicating p21 expression. We report that high concentrations of control monoclonal antibodies duplicated exactly the immunohistochemical staining pattern of RAP-5 on formalin-fixed tissue sections and that RAP-5 staining was not competitively inhibited by either the portion of p21 it was raised against or by the thyroglobulin-conjugated peptide used as immunogen. In an enzyme-linked immunosorbent assay, RAP-5 also showed greater nonspecific binding to poly-L-lysine and to polystyrene wells than did control antibodies. We conclude that RAP-5 binding to formalin-fixed tissue sections is nonspecific and not indicative of p21 expression.