- The effects of the H‐ras oncogene on fibroblast cell tumorigenicity and immunogenicity was studied in transfectants of the BALB/c 3T3 clone A31 fibroblastoid cell‐line. Cells that were transfected with MC29‐LTR‐H‐ras (98/6) or MC29‐LTR‐v‐myc + H‐ras (98/4v) and were inoculated into syngeneic BALB/c mice were tumorigenic in 100% and 60% of animals respectively. By contrast, transfectants containing the pSV2neo plasmid alone (98/1) displayed normal characteristics both in vitro and in vivo. Inoculation of mice with mitomycin‐C‐treated 98/1 or 98/4v cells induced an effective protective immunity to a challenge of live 98/4v cells, and a partial immunity against 98/6 cells. Mitomycin‐C‐treated 98/6 cells failed to render immunity against a challenge of either 98/6 or 98/4v cells. To correlate immunogenicity and tumorigenicity of the different cell types with cell‐surface‐antigen expression, we prepared MAbs against 98/4v cells in syngeneic mice. Immunohistochemical and immunoblot analysis revealed that MAbs 102 and 104 recognized 2 protein bands of 70 and 45 kDa respectively, which were expressed predominantly in 98/1 and 98/4v cells. A third immunoreactive protein band of 44 kDa that reacted with MAb 6 was expressed at a similar cell‐surface density on all cell types. Cell‐differentiation‐inducing agents, such as DMSO, retinoic acid or sodium butyrate, were all found to induce 98/6 cell flattening and morphological changes toward a normal phenotype that were followed by up‐regulation of the 70‐ and 45‐kDa antigens. The results suggest that regulation of expression of the 70‐ and 45‐kDa molecules is affected by H‐ras, and that expression of these cell‐surface molecules may be relevant to tumor cell immunogenicity. © 1992 Wiley‐Liss, Inc.