The concentration of secretory immunoglobulin A (IgA) in the vaginal fluid of 20 untreated post-menopausal women (aged 75.4 +/- 1.4 years) was compared with the levels in a group of 20 post-menopausal women (aged 73.2 +/- 1.5 years) treated with oral oestriol (E3) (2 mg/day) and a further group of 20 healthy, non-pregnant, fertile women (aged 28.2 +/- 1.8 years). Secretory IgA was determined using a paper disc modification of the single radial immunodiffusion technique. The vaginal concentration of IgA in the untreated women was 41.5 +/- 5.7 mg/l, which was higher than that recorded in those treated with E3 (20.8 +/- 5.7 mg/l, P less than 0.05) and in the fertile women (16.4 +/- 3.7 mg/l, P less than 0.01). Epithelial cells predominated in the wet smear preparations obtained from the women who received E3 and the fertile women, while a predominance of leucocytes was observed in the untreated women. Lactobacilli were found more frequently in the vaginal flora of the women in the E3 group (P less than 0.001) and the fertile women (P less than 0.001) than the untreated group. Faecal-type bacteria were seen more frequently in the vaginal flora of the untreated women than the E3 group (P less than 0.01) or the fertile women (P less than 0.001). The possible implications of the present findings with regard to the individual variation in the intensity of symptoms experienced by women suffering from the oestrogen deficiency syndrome are discussed.