17beta-Estradiol-3-monobenzoate injected s.c. in mice led to a dose-dependent stimulation of the global reticuloendothelial clearance function. Furthermore, a significant depression of serum hemagglutinin levels, being independent of the anitgen dose, was noticed. Although a slight decrease of antigen-sensitive (memory) cells was revealed, estradiol treatment before priming failed to impair immunological memory as assayed by hemagglutinin titers in the secondary reaction. Estradiol administration was followed by charakteristic organ weight changes (liver, thymus, adrenals). According to the literature it is suggested that estradiol, like diethylstilbestrol, is capable of depressing humoral antibody formation by differential effects on the reticuloendothelial system.