Upper genital tract isolates at delivery as predictors of post-cesarean infections among women receiving antibiotic prophylaxis. The introduction of antibiotic prophylaxis for cesarean delivery has decreased the risk of postpartum endometritis and wound infection, but factors that contribute to prophylaxis failure are not understood. To determine factors that might contribute to postpartum infections following antibiotic prophylaxis, we cultured amniotic fluid, decidua, and chorioamniotic membrane specimens for anaerobic and facultative bacteria and for genital mycoplasmas at cesarean delivery. Women were assessed daily for the development of infections, and if endometritis developed, a protected endometrial culture was obtained. Postpartum endometritis developed in 16 and wound infection in four of 102 women. Infection rates were similar for women receiving cefotetan (N = 50) or cefoxitin (N = 52) for prophylaxis. The isolation of group B streptococcus (P less than .001) or Enterococcus faecalis (P = .03) from the upper genital tract at delivery was significantly associated with postpartum endometritis. Antibiotic-resistant organisms (other than enterococci) were recovered uncommonly at delivery or with postpartum infections. Group B streptococcus was susceptible to the prophylactic agents used, suggesting that virulence factors other than antibiotic resistance are important for the development of postpartum endometritis. Group B streptococcus, E faecalis, and bacteria associated with bacterial vaginosis were recovered from the endometrium at the time of postpartum endometritis. 