Cells from the spleens and thymuses of BALB/c mice whose Moloney sarcoma virus (MSV)-induced, primary sarcomas have regressed 2-3 months earlier ("MSV regressors") or are in the process of regressing can, when adoptively transferred to syngeneic mice given MSV at the age of 20 days, prevent the natural regression of the MSV sarcomas in the recipient mice. The cells responsible for this tumor-enhancing effect express the Thy 1 marker. They are not demonstrable in the thymuses of normal untreated mice or in mice that have either been immunized against or are bearing methylcholanthrene-induced sarcomas. The tumor-enhancing cells are not destroyed after administration of 400 rads (1 rad = 1.00 x 10(-2) J/kg) of whole body radiation. However, the effect of the irradiated cells is seen only in the presence of a nonirradiated T-cell population, represented in the thymuses of normal control mice, with which we postulate that they interact. Studies on a transplantable, chemically induced, murine leukemia virus antigen-negative sarcoma, MCA-1460, further support the concept that relatively radioresistant thymus cells from immune mice can enhance tumor outgrowth by interacting with radiosensitive T cells that are present in nonimmune mice.