Studies on the tumorigenicity of Vero kidney cells of Cercopithecus aethiops monkey origin were extended to various passage levels of BSC-1 aneuploid cells and to low passage CV-1 diploid cells (derived also from C. aethiops monkey kidney). It was found that BSC-1 cells -- like Vero cells -- showed increased tumorigenicity with increasing passage level in antithymocyte globulin (ATG) treated newborn rats and in nude mice. Cells passaged over 250 times in cultures formed invasive adenocarcinomas in newborn rats. Their malignant tumor growth was further demonstrated around the 500 passage level when tumor metastases were detected in the lungs of four of the 14 inoculated rats. Vero cells induced such lung metastases in rats already at passage 227. CV-1 diploid cells at low passage level produced small nodules of epithelioid cells in newborn rats at 6th day after inoculation that had disappeared by the 21st day, and caused no local invasion nor lung metastasis. In vitro tumorigenicity tests on BSC-1 and CV-1 cells, using chick embryo skin, human muscle and colony formation in agarose, confirmed the animal test results. The results of this study indicate that BSC-1 and Vero cell lines at low and high passage levels may prove to be useful tools to study the molecular basis of malignancy.