The human ST5 gene is expressed as 4.6, 3.1 and 2.8 kb transcripts encoding putative 126, 82 and 70 kDa proteins that function in the MAP kinase signaling pathway in transient expression assays. Expression of the 2.8 kb transcript correlates with reduced tumorigenicity in HeLa-fibroblast hybrids, suggesting a role in tumor suppression. We now report the detection of ST5 proteins in cellular extracts, demonstrate specific expression of p70 in non-tumorigenic HeLa-fibroblast hybrids, extend the correlation between p70 expression and cellular morphology to a wide variety of cell lines, and provide direct evidence that p70 can effect changes in cell growth and morphology. ST5 proteins were identified in extracts of human, mouse and simian epithelial cells and fibroblasts, but were absent from lymphoid cells. Transfection of the 2.8 kb cDNA into a p70-negative mouse fibroblast line yielded stable transfectants with a flattened, less refractile morphology relative to controls. The p70 expressing clones had initial growth rates similar to those of control cells but their saturation density was reduced threefold, suggesting a restoration of contact-regulated growth. In conjunction with previous findings, these results suggest that ST5 proteins participate directly in events affecting cytoskeletal organization and tumorigenicity.