Because of the undesirable side effects associated with the use of 8-methoxypsoralen and long-wave ultraviolet A (UVA) radiation in the treatment of skin disorders such as psoriasis, the use of monofunctional psoralens, which are less erythemogenic, less mutagenic, and generally non-phototoxic, has received considerable attention. Little is known, however, about the immunosuppressive properties of monofunctional psoralens. The purpose of this study was to examine the effect of parenteral administration of a monofunctional psoralen, angelicin, plus exposure to UVA radiation on the immune response. Injection of angelicin followed by exposure to UVA radiation significantly suppressed delayed-type hypersensitivity to alloantigen in a dose-dependent fashion. Similarly, the capacity of spleen cells from the angelicin and UVA-treated animals to proliferate to alloantigen was significantly suppressed. The suppression was specific for the alloantigen used to sensitize the angelicin and UVA-treated animals and was associated with the appearance of splenic antigen-specific suppressor T lymphocytes. These data demonstrate that the effect of systemic administration of a monofunctional psoralen followed by UVA exposure on the immune response is similar to that seen following the injection of bifunctional psoralens. These findings also suggest that the severe skin phototoxicity associated with the use of a bifunctional psoralen and UVA radiation is not necessary for the induction of systemic immuno-suppression. Furthermore, the induction of systemic antigen-specific immunosuppression by angelicin plus UVA, without overt skin phototoxicity, suggests the possibility of using this and related compounds to specifically inhibit unwanted immune reactions.