p53 is a cellular protein whose levels are some 1500-2000 times higher in adenovirus and SV40-transformed human cell lines than in homologous nontransformed cells. Monoclonal antibodies have been produced that detect p53 of primate origin but not of rodent origin. These monoclonal antibodies have been employed to study the properties of p53 antigens from human cell lines. Human p53 proteins of at least five different apparent molecular-weight classes in SDS-polyacrylamide gels have been detected. In some cell lines, at least two distinct molecular-weight species are expressed and these two forms have similar or identical partial peptide maps. Both molecular-weight forms can be resolved into seven or eight species upon isoelectric focusing in a two-dimensional gel system. There is also some indication of differences in the partial peptide maps of human p53 antigens derived from different human transformed cell lines. A radioimmunometric assay was employed to study the steady-state levels of oligomeric p53 in normal and transformed cell lines. Antibody affinity chromatography has been employed to purify p53 protein which was then used to quantitate the steady-state levels of p53 in different human cell lines. Normal cells had little or no detectable p53 antigen. Transformed cells or tumor-derived cell lines varied between no detectable p53 protein and 450 micrograms of p53 protein/g of cellular protein (in SV80 cells). There was a great diversity in the levels of p53 antigen in human cells. SV40- and adenovirus-transformed cells had by far the highest levels of p53 antigen. These are the viruses whose tumor antigens have been shown to be associated in an oligomeric complex with p53 in transformed cells. Eleven out of fifteen human tumor derived or transformed cell lines contained greater than five-fold higher levels of p53 antigen than normal human cells.