Effects of immunization on the growth of the early passages of two methylcholanthrene induced mouse tumors, MC57 X and MC57 Y, were different. For tumor X, immunization resulted in complete protection, while tumor Y was enhanced. The enhancing effect was individual tumor specific. The spleens of tumor bearing animals were enlarged and the proportion of blasts, EAC- and EA-rosettes were increased. This was the case for the T cell enriched population also. In lesser extent, such findings were also obtained in the lymph nodes. The spleens of control and tumor bearer animals were tested for effect on the in vitro growth of tumor cells. From both sources non-adherent subpopulations enriched in C3 receptor bearing cells substantially reduced the number of tumor cells. Using the T cell fractions, only those derived from tumor bearer spleens were active. The in vitro effects on the growth of the two tumors were cross-reactive. T cells exhibiting growth inhibition in vitro were detectable 13 days after subcutaneous inoculation, when the tumor was already measurable, while earlier the T cells enhanced the growth. The outcome of the in vitro tests reflected only the tumorous status of the animal and did not relate to preimmunization.