A microelectrode was used to measure vaginal pH in a longitudinal study during pregnancy in 30 women, 28 of whom were delivered at term. Excluding pH readings made in the presence of pathogenic organisms, 120 readings were taken. The mean vaginal pH of all subjects was 4.16 (SD 0.41). Analysis of variance showed no significant within-subject variation of pH during pregnancy. Two women had spontaneous preterm deliveries (less than 37 weeks) and their mean vaginal pH during pregnancy was 5.05 and 5.06. Eight women admitted in preterm labour, who then gave birth, had a mean pH of 5.43 (SD 0.94) which was significantly higher than the mean pH of 10 women in labour at term (4.58, SD 0.59). The relation between bacterial growth in vitro and the surrounding pH was also examined. Escherichia coli, a potential pathogen, behaved differently from both Lactobacillus (a commensal organism) and Candida albicans. The clinical observations show a trend towards higher vaginal pH values in association with preterm labour.