Suppression of muscle type isoforms of tropomyosin (TM) is a common biochemical event in malignantly transformed cells. To evaluate the role of TM proteins and isoform specificity in cellular transformation, cDNAs that consist of coding sequences of TM1 (product of beta gene) and TM2 (product of alpha gene), but lacking untranslated regions (UTRs), have been expressed separately in DT (v-Ki-ras transformed NIH3T3) cells, and elevated levels of the corresponding proteins were detected. DT cells which over express TM2 manifest growth in soft agar. Elevated levels of TM1 protein in DT cells resulted in flattened cell morphology and complete abolition of anchorage independent growth. Tumorigenesis in athymic nude mice was observed in the absence of transduced TM1 mRNA. Thus, expression of TM1 protein is sufficient for tumor suppression: the UTRs of TM1 are not required for the tumor suppressive effects. Expression of TM2 protein, on the other hand, has no effect on the transformed phenotype of DT cells. These data indicate that isoforms 1 and 2 of TMs perform distinct physiological roles.