Tritiated thymidine incorporation by Lewis rat spleen cells stimulated in mixed lymphocyte cultures with F1 hybrid spleen cells was suppressed by treatment of the cultures with 5-bromodeoxyuridine and light. Reduction of the in vitro proliferative responses was accompanied by similar abrogation of the capacity of cultured cells to induce local graft-versus-host (GVH) responses in susceptible F1 recipients. This effect was not observed with unstimulated, but similarly treated cells. The findings establish a relationship between cells responding in mixed lymphocyte cultures and cells mediating GVH reactions; both responses are diminished by a treatment lethal to replicating cells. These studies suggest avenues for further investigation of transplantation of immunocompetent cells. © 1972.