One of the current criteria for evaluating the acceptability of cell lines for use in vaccine production is lack of tumorigenicity. Vero cells represent an example of a class of cells known as continuous cell lines. They were derived from African green monkey kidney, and their growth properties and culture characteristics have many advantages over other cell substrates for use in vaccine production. We have tested Vero cells for tumorigenicity in nude mice and in a human muscle organ culture system, and found a significant increase in their tumorigenic potential with increasing passage numbers. Cells at passage 232 and higher produced nodules in all nude mice inoculated. Histologically the nodules were well defined, anaplastic tumors, which exhibited some of the characteristics of renal adenocarcinomas. In about 6 to 8 days all of the nodules began to regress. Data were obtained that suggested an immune mechanism was the basis for the regression phenomenon.