Susceptibility to infection due to intra-amniotic type III group B streptococcal infection was studied in 27 rhesus monkeys. Sera from mothers and their offspring were tested to determine the concentration of antibody to the native type III group B Streptococcus antigen. Among 17 controls there was a statistically significant association between the concentration of maternal antibody prior to infection and both the neonatal survival rate and survival time (P less than 0.05). Neonatal survival was decreased to less than or equal to 6 hours (P = 0.005) if the maternal antibody concentration was less than 0.5 micrograms/ml. Modified immune serum globulin was given intravenously to the mothers prior to intra-amniotic infection with (five animals) or without (five animals) neonatal modified immune serum globulin. Neither of the modified immune serum globulin groups demonstrated a significant reduction in the neonatal mortality rate; however, the addition of the modified immune serum globulin provided protection against rapid neonatal death among those animals born to mothers which had low or no detectable antibody. All maternal groups developed a significant increase in the concentration of antibody in postpartum sera. These results indicate that both naturally acquired and passive (modified immune serum globulin) antibodies to type III group B Streptococcus antigen are partially protective against intra-amniotic infection.