The cervico-vaginal secretions of 58 women were shown to contain IgG and IgA globulins in high concentration. The ratio IgG/IgA was found to be 1.5. IgM globulin was demonstrated only occasionally. Agglutinating antibodies to Candida albicans were present in 48% of the samples and antibodies to E.Coli in 77%. The opsonizing property of genital tract secretions against Enterococcus, Kleb-siella, Proteus vulgaris and Pseudomonas was found to be present in 207 tests (79%). Absorption of the material with a rabbit anti-human y-globulin anti-serum removed almost all the opsonizing activity. Lysozyme and complement were shown to enhance the phagocytic properties of these external secretions. Local immunological mechanisms are likely to play a significant role in the protection of human genital tract against infection.