The isolation and construction of a complete human p53 cDNA and subsequent expression in monkey cells is described. A set of new anti-(human p53) monoclonal antibodies has also been obtained and used to show the expression of the human p53 cDNA in cos-1 cells. These antibodies enable the specific detection of human p53, which is synthesised in the presence of p53 from other species. Fusion proteins of p53 with β-galactosidase were used firstly as antigen and secondly, in conjunction with competition assays, to localise the determinants recognised by the antibodies. At least two previously unrecognised epitopes are involved and two of the antibodies are human-p53-specific. The epitopes are denaturation-resistant and the antibodies are, therefore, valuable for immunoblotting as well as immunoprecipitation and enzyme-linked immunoassay. Transfection of plasmids containing complete human p53 cDNA into monkey (cos-1) cells cause expression of human p53 recognised by the monoclonal antibodies. Control plasmids did not induce immunoreactive protein.