Normal, healthy human volunteers and patients with proved history of non-melanoma skin cancer have been tested for their capacity to develop contact hypersensitivity to dinitrochlorobenzene (DNCB) following exposure of buttock skin to acute, low-dose ultraviolet B (UVB) radiation. Using a radiation protocol that achieves virtually complete depletion of normal-appearing Langerhans cells from irradiated skin, it was learned that approximately 60% of healthy volunteers developed vigorous contact hypersensitivity (CH) when 2000 micrograms DNCB was painted on the irradiated site. These individuals were designated UVB-resistant, and were distinguished from other individuals, designated UVB-susceptible, who failed to develop contact hypersensitivity following an identical treatment protocol. It was then discovered that virtually all (92%) skin cancer patients exposed to UVB and DNCB failed to develop CH, i.e., were UVB-susceptible. In subsequent experiments, epicutaneous application of 2000 micrograms DNCB to unirradiated skin of UVB-susceptible individuals revealed a further distinction between normal persons and skin cancer patients. Approximately 45% of the latter (and none of the former) remained unresponsive (failed to develop contact hypersensitivity following this second attempt at sensitization), implying that they had been rendered immunologically tolerant. These tolerant individuals responded normally to the unrelated hapten, diphencyprone. We conclude that human beings resemble inbred strains of laboratory mice in that some individuals are UVB-susceptible, whereas others are UVB-resistant. Because the incidence of UVB-susceptibility was significantly higher in skin cancer patients, and as specific unresponsiveness could be demonstrated only in these patients, we propose that UVB-susceptibility, as we define it in this hapten system, may be a risk factor for the development of skin cancer.