Monoclonal antibodies were used to localize and characterize leukocytes and cytokines in chorionic villi of normal placenta and complete mole. OKT4a reacted with Hofbauer cells in first-, second-, and third-trimester placentas. HLA-DR was expressed on second- and third-trimester placental Hofbauer cells but was not detected in the first trimester. The cytokines, γ-interferon, granulocyte-macrophage colony-stimulating factor, interleukin-1α, and tumor necrosis factor α, were all detectable on Hofbauer cells in first-, second-, and third-trimester placentas. None of the tested cytokine antibodies reacted with cellular constituents in complete molar chorionic villi. The absence of cytokine-positive cells in molar chorionic villi may be related to the hyperplastic and unregulated growth of molar villous trophoblast. © 1990.