Two synthetic peptides corresponding to residues 1-20 and 10-20, respectively, of one type of a cellular protein called "p53" have been linked to a carrier protein and injected into rabbits to raise antibodies. The antibodies obtained were capable of reacting with the native protein, as judged by an enzyme-linked immunosorbent assay, protein A-linked staining of immunoblots after NaDodSO4 gel electrophoresis, and immunoprecipitation. The immunoassay titers against the protein were lower for these antibodies than for antisera derived from immunization with purified p53. However, staining with the immunoblot method showed that the antipeptide antibodies against p53 were uniquely specific. The data suggest that at least two different types of p53 molecules occur. The cellular protein previously isolated from human cells transformed by Epstein-Barr virus and from murine tumors induced by methylcholanthrene appears to be larger than the p53 reported in relation to simian virus 40- or adenovirus-transformed cells and to some other tumors. Some interrelationships have not been excluded, but it is clear that the two protein molecules do not behave identically. The reactions of the antipeptide antibodies with the intact protein have implications in regard to protein conformations. The strict specificities of such antibodies allow the generation of distinct sets of reagents useful for quantitation, purification, and cloning.