Uterine fluid obtained from estrogen primed rats (adult-ovari-or immature) has an antimicrobial effect on certain bacterial, fungal and viral species when combined with iodide ions and H202. Iodide ions can be replaced by bromide ions and reagent H2O2 by glucose and glucose oxidase or by a H2O2-generating microorganism such as Lactobacillus acidophilus. A heat stable, low molecular weight inhibitor or inhibitors of the uterine fluidiodide-H2O2 antimicrobial system also is present in uterine fluid. Reduced glutathione, cysteine, NADH, NADPH and ascorbic acid can inhibit the antimicrobial system. Uterine fluid exhibits peroxidase activity and its antimicrobial effect in the presence of iodide and H2O2 is believed to be due to this enzyme. The possible presence of the peroxidase, halide, H2O2 antimicrobial system in human vaginal or uterine secretions is considered.