Eighty-six percent of 707 β3-hemolytic streptococci isolated in a general hospital and excluded by presumptive tests from groups A and D were identified serologically as group B. More than 70% of the group B isolates were from urine cultures, the female genital tract, or newborn infants. Types III and II were the most common group B serotypes from most sources. However, types la, Ib, and Ie were more commonly isolated from the respiratory tract than from other sites, and type III was responsible for most serious neonatal infections. All group B streptococci were sensitive in vitro to comparable levels of penicillin G, ampicillin, and cephalothin and were highly resistant to kanamycin. Seventy-two percent were resistant to tetracycline but only 1%-2% to erythromycin, c1indamycin, and chloramphenicol. Despite consistent sensitivity to penicillin G, the minimal inhibitory concentrations were significantly higher for group B than for group A streptococci.