The presence of group-B streptococci in the urine of pregnant women seems to be associated with preterm labour. Urine samples from 4122 women at 27-31 weeks' gestation were examined for bacteria. Group-B streptococci were found in the urine of 69 women. In a double-blind, controlled study these patients were given either penicillin (10(6) IU three times daily for 6 days; 37 patients) or placebo (32 patients). The rates of primary rupture of the membranes (11% v 53%; p less than 0.001) and preterm labour (5.4% v 38%; p less than 0.002) were significantly lower in the penicillin group than in the placebo group. These results suggest that treatment and follow-up to prevent recolonisation in pregnant women with group-B streptococci in the urine may reduce the frequency of preterm labour in these patients.