The concentrations of immunoglobulin A (IgA) and immunoglobulin G (IgG) were estimated in cervical mucus from 115 patients attending a clinic for sexually transmitted diseases. The patients were divided into two groups; those using combined estrogen/progestogen oral contraceptives, and those with presumed normal ovulatory cycles. Gonorrhea, trichomoniasis, candidosis, and herpes genitalis were diagnosed by conventional smear and culture techniques, and the two groups were subdivided according to these diagnoses. Gonorrhea, trichomoniasis, herpetic, and nonspecific cervicitis all caused marked increases in the mean concentrations when compared with a control group of uninfected patients with natural cycles (P = less than 0.01). Patients with candidosis and contacts of men with nonspecific urethritis showed a lesser rise. IgG/IgA ratios lower than that of serum suggested a considerable locally produced contribution of IgA. Oral contraception with the combined pill also caused a significant increase in mean IgA and IgG levels even in the absence of local infection (P = less than 0.01). an increase in the IgG/IgA ratio of this group may indicate that the hormonal effect was manifest through increased serum transudation. IgM was also detected more commonly in patients taking the pill. The marked effect of local disease on immunoglobulin levels in cervical mucus which occurs even in asymptomatic patients emphasizes the importance of screening for infection when studying these secretions.