Contrary to previous reports, human granulocytes are capable of causing transformation of allogeneic lymphocytes. Pretreatment of granulocytes with mitomycin C results in nonstimulation, but untreated granulocytes are capable of stimulating lymphocytes. Nonstimulation with mitomycin C-treated granulocytes may result from the reduced viability of the cells. It is concluded that granulocytes possess alloantigens that stimulate lymphocytes in vitro.