In a population of pregnant women, the prevalence of group-B streptococcal carriage was relatively low. During the 3rd trimester of pregnancy 5-6% of women haboured group-B streptococci and 8-3% were positive at the onset of labour. Some 42% of women who gave positive cultures in labour had given negative cultures during the 3rd trimester and 19% of women who were positive during late pregnancy were culture-negative in labour. The conversion of culture status observed in these women suggests that carriage may be intermittent or that new acquisition of genital-tract streptococci may occur in late pregnancy. The unpredictability of conversion diminishes the reliability of a single culture taken during the 3rd trimester of pregnancy.