Summary Both ultraviolet (UV) irradiation and cw-urocanic acid (UCA) are reported to be associated with the suppression of contact hypersensitivily (CHS) and the induction of a soluble factor which suppresses leukocyte adherence inhibition (LAI) assays in vitro. The cellular origin of the suppressor factor (SF) has now been investigated in vivo and in vitro, using monoclonal antibodies against lymphocyte surface markers to deplete certain ceil populations, namely T lymphocytes beanng the L3T4 (helper) and Lyt-2 (suppressor) markers. Depletion of Lyt-2+ cells from irradiated mice in vivo and from spleen cell cultures in vitro led lo the elimination of detectable levels of SF. Depletion of L3T4+ cells had no such effect. Similarly, Lyt-2+ cells (but not L3T4+ cells) were shown to be necessary for the production of SF by normal spleen cells cultured with m-UCA. These data suggest that the production of SF following UV irradiation may be related to the action of m-UCA on Lyg-2 + lymphocytes.