We report here on the development of enzyme-linked immunosorbent assays (ELISAs) for antibodies to types II and III group B streptococci. Streptococcal antigens were prepared by trichloroacetic acid extraction and fractional alcohol precipitation. Microtiter wells were coated with antigen in 0.1 M carbonate buffer at pH 9.6. Lyophilization was found to be an essential step for efficient binding of the streptococcal antigens. After incubation with antibody-containing rabbit serum, bound antibody was detected with peroxidase-labeled goat anti-rabbit immunoglobulin G. Optimal antigen concentrations were 200 micrograms/ml for type II and 100 micrograms/ml for type III. An ELISA is also described that uses intact bacteria as antigen. Hyperimmune rabbit serum reacted at a titer in excess of 512 against trichloroacetic acid-soluble antigen and 4,096 against whole bacteria. Sera from human subjects were also tested. Most human sera contained antibody which bound to intact bacteria but not to trichloroacetic acid-solubilized streptococcal antigens. Antibody titers in human sera against intact bacteria correlated very well with opsonic antibody activity measured in a chemiluminescence assay.