Expression of the growth suppressor gene p53 in Schizosaccharomyces pombe causes growth arrest which can be suppressed by overexpression of a yeast gene termed tms1. The tms1 gene encodes a 37 KDa protein which binds to p53 in vitro and in vivo when p53 was expressed in fission yeast cells. By using antibodies against recombinant tms1 protein we analyzed mammalian cells for the expression of a tms1 related protein. Human, monkey and rat cells exhibited a bright nuclear fluorescence when incubated with the tms1 specific antibody. In addition, we were able to specifically immunoprecipitate a 42 KDa protein which forms complexes with the p53 protein. The specificity of these complexes was demonstrated by coimmunoprecipitation using different monoclonal antibodies against p53, antibodies against yeast tms1 as well as by Western blot analysis. Thus, we found that the mammalian tms1 related protein represents a new p53 binding protein.