The antifungal effects of two commercially available spermicidal gels (Conceptrol, produced by Ortho Pharmaceutical, Raritan, N.J., and Koromex, produced by Schmid Laboratories, Little Falls, N.J.) as well as pure nonoxynol-9 and boric acid (both components of vaginal contraceptive products) were tested against 50 clinical yeast isolates by the agar dilution method. The formulated products exerted comparable dose-dependent inhibitory effects against all yeasts tested. A 3-fold dilution of the formulated spermicidal products inhibited 90% of the yeast strains tested. To determine if the antifungal effect was due to the spermicidal detergent nonoxynol-9, this compound was tested for antifungal activity but was completely ineffective against Candida albicans in concentrations up to 10%. Boric acid, present in at least one of the products (Koromex), inhibited representative yeasts at a concentration of 0.4%. The relationship of pH and oxygen tension to inhibition by the commercial spermicides was also investigated. The pH values tested ranged from 4 to 7 and had little effect on inhibition; anaerobiosis at pH 7 slightly reduced the inhibitory activity of Conceptrol gel.