Penicillin treatment and antibody response were studied using a rhesus monkey model for intraamniotic infection with type III group B streptococci (T3GBS). Acute and convalescent phase sera from mothers and their offspring were tested with a radioactive antigen-binding assay to determine the concentration of antibody to the capsular T3GBS antigen. The frequency of placentitis was significantly lower in penicillin-treated animals (3 of 8) than in controls (10 of 10; P less than .01). The penicillin group also had a significantly lower neonatal mortality (1 of 9) than controls (6 of 10; P less than .05). Both groups of rhesus mothers developed a significant increase in concentration of antibody to T3GBS, but the antibody response was of lesser magnitude in the penicillin-treated group. This experimental model appears to be useful for studying both therapy for intraamniotic infection and the humoral immune response to infection with T3GBS.