Murine alpha-fetoprotein (AFP) is a major protein in amnionic fluid and perinatal sera. AFP has been postulated to contribute to the immunologic hyporesponsiveness of the fetus and neonate, as well as protecting the fetus from rejection by the mother. We now report that AFP acts in vitro to inhibit macrophage expression of cell surface Ia antigens, the class II glycoproteins of the major histocompatibility gene complex. In culture, macrophages incubated with a T cell lymphokine developed Ia as detected by either immunofluorescence or a cell radioimmunoassay. Both mouse amnionic fluid and AFP inhibited the expression of Ia in a dose-dependent manner. Five preparations of AFP derived from three sources--day-old neonates, amnionic fluid, and a hepatoma cell line--were effective. The concentration of AFP that inhibited by 50% the expression of Ia was about 10(-6) M. Mouse amnionic fluid and AFP did not affect the viability of the macrophage nor was the surface expression of H-2K and C3 receptors affected. The inhibitory effect of AFP did not depend on the presence of T cells in the culture. AFP did not appear to inhibit the direct interaction of lymphokine with macrophages; AFP did inhibit if given days after a pulse of lymphokine. The concentrations of prostaglandins carried by AFP were insignificant and could not explain the inhibitory effects.