Irradiation of BALB/c mice with FS40 sunlamps induces susceptibility to challenge with syngeneic UV-induced fibrosarcomas that otherwise would be rejected immunologically. Plastic filters were used to remove various wavelengths from the radiation source to identify the active waveband. Wavelengths above 315 nm (those transmitted through a mylar filter) were ineffective in producing tumor susceptibility, even when the total amount of energy applied was the same as that emitted by the unfiltered FS40 sunlamps. Removal of wavelengths below 275 nm (with a polystyrene filter) did not reduce the activity of the radiation. Alteration of the dose-rate of the radiation by as much as a factor of 10. by irradiating animals through neutral density filters, did not change the proportion of tumor-susceptible mice, suggesting that there is reciprocity with regard to dose-rate and time of UV exposure. Cell transfer studies were used to test whether similar immunologic mechanisms were responsible for the equal susceptibility to tumor challenge of mice given continuous UV exposure (one 12-h treatment) or fractionated exposures (twelve 1-h treatments). With both treatment regimens, tumor susceptibility could be transferred to X-irradiated recipients with lymphoid cells, provided that sufficient time had elapsed after the single treatment.