The Human gene map will be extended by identifying human polypeptides among total polypeptides from rodent-human hybrid cell lines that carry only one human chromosome or a fragement thereof. 3H-labeled hybrid cell polypeptides are mixed with 14C-labeled parental cell polypeptides and subjected to high- resolution two-dimensional polyacrylamide gel electrophoresis; putative human gene products, which are labeled only with 3H, are then located by double-label autoradiography. Pilot studies with a hybrid cell line carrying human chromosome 12 have shown the effectivenes of the method; further work with hybrids carrying all or part of chromosomes 11 or 12 is planned. It is expected that the functional identity of many of the polypeptides identified initially only as electrophoretic entities will become known, as a standard human polypeptide two-dimensional gel map is developed over a period of years. The same techniques will be used to compare fibroblast polypeptides from different human individuals, in order to make a new and comprehensive estimate of average heterozygosity, as expressed in electrophoretically separable polypeptides.