In a population of 1011 puerperal women, the significance of rectovaginal colonization by group B streptococci during pregnancy with respect to infective puerperal morbidity was analyzed. Patients who were found to be carriers during pregnancy (121) were randomly divided into two groups: women who received ampicillin during delivery (500 mg i.v./6 h) and patients without chemoprophylaxis. Compared with the non-carriers, the carrier patients without prophylaxis had a significant increase in the mild puerperal infective morbidity, when defined as the proportion of women with an index fever greater than or equal to 10 (10.6% vs. 25%). However, the increased incidence among the carrier women of premature rupture of the membranes and of other possible morbidity factors made it impossible to identify the role of group B streptococci. Mild puerperal infective morbidity in the carrier women who received prophylaxis was lower than in those without prophylaxis and very similar to that of non-carrier women. It is concluded that the use of chemoprophylaxis to prevent neonatal sepsis would probably be followed by a reduction in infectious puerperal morbidity.