Tamm-Horsfall glycoprotein (THG) purified from pregnancy urine was found to stimulate normal human mononuclear cell (MNC) proliferation at a concentration greater than 10 micrograms/ml. This stimulation was non-specific because the percentage of B and T cell subpopulations including CD20, CD3, CD4, CD8 and CD4/CD8 ratio was not changed by THG. THG not only bound to human mononuclear cells but depolarized the membrane potential, increased 22Na+ uptake and enhanced the expression of IL-2R and HLA-class II antigens on these cells. The concentrations of sIL-2R, sCD4 and sCD8 in the THG-stimulated MNC culture supernatants were significantly increased compared with control supernatants. In addition, overnight incubation of THG (5-50 micrograms/ml) with MNC dose-responsively enhanced the syntheses of IL-1 beta, IL-6 and TNF-alpha by monocytes, with a maximal effect at 25 micrograms/ml. This monokine releasing activity of THG could be neutralized by a specific antibody against THG. When monocytes/macrophages were depleted from mononuclear cells by incubating with lysosomotropic methyl ester of L-leucine, THG retained the capability of stimulating lymphocytes proliferation but to a lesser degree. These results suggest that urinary THG activates monocytes to synthesize large amount of monokines through its membrane effect. The released monokines subsequently stimulate lymphocytes expressing IL-2R and HLA-class II antigens and finally lead to cell proliferation.