A cloned strain of bovine vascular endothelial cells with a finite in vitro lifespan was treated with benzo(a)pyrene (BP) after approximately 75% of its lifespan was completed. Untreated cultures of this strain senesced upon serial subcultivation and contained large, nondividing cells. In three out of seven trials, BP treatment produced transformed lines with altered phenotypic characteristics. The transformed cells appeared in the cultures concomitant with the senescence of the parent cells. All transformed cell lines examined exhibited indefinite lifespans and altered karyotypes. Two of the lines retained most of the characteristics of normal endothelial cells, except that one became aneuploid and the other polyploid. Neither of these lines formed tumors when inoculated into nude mice. The remaining two lines retained mostly diploid karyotypes, but a high percentage of cells contained Robertsonian translocations. In one line cell volume was markedly reduced. In addition, these lines grew in multilayers, were anchorage independent, and proliferated in medium containing 0.5% serum. When 10(7) cells of these lines were injected into nude mice, tumors appeared within 1 week and were identified as malignant hemangioendotheliomas of bovine origin.