The immunogenicity of KMT-17 fibrosarcoma cells which had been xenogenized by infection with FV was compared to that of KMT-17 cells which had been admixed with BCG. We report here that 10(5) and 10(6) KMT-17 cells also grew progressively to kill rats, but when 10(5) KMT-17 cells were administered with BCG the tumor cells did not grow in the majority of rats. The strength of immunogenicity (ETD50), as measured by the number of immunizing cells required for a suppression of growth of 10(7) KMT-17 cells in 50% of the rats, was 2.1 X 10(3) for FV-KMT-17 and 36.3 X 10(3) for BCG + KMT-17. The tumor cell dose (LTD50) which was required to kill 50% of the rats immunized with 10(5) FV-KMT-17 was more than 10,000 times higher than that found in normal rats, whereas the number of tumor cells required to kill 50% of the rats immunized with the same number of BCG + KMT-17 was only 3,680 times higher than the amount found in normal rats. Thus the immunogenicity of FV-KMT-17 is much stronger than that of BCG + KMT-17. The difference in immunogenicity between the two vaccines was also observed in the tumor-neutralizing activities of spleen cells obtained from rats which had been immunized with both vaccines, as measured by a Winn assay. Moreover, the antitumor activity of spleen cells from rats immunized with FV-KMT-17 was concentrated in the carrageenan-resistant and plastic nonadherent cells, while that of spleen cells from rats immunized with BCG + KMT-17 was observed in carrageenan-sensitive and plastic adherent cells as well as in nonadherent cells. The involvement of different effector cells indicates that different mechanisms operate in the antitumor resistance in rats immunized with either FV-KMT-17 or BCG + KMT-17.