The effect of various doses of estradiol on genital tract infection by the chlamydial agent of guinea pig inclusion conjunctivitis (GPIC) was investigated in ovariectomized guinea pigs. Prolongation of infection, as determined by chlamydial inclusion counts of cells in Giemsa-stained smears of vaginal scrapings, was observed in animals receiving daily doses of 1.0, 10.0, 100.0, or 1000 micrograms of estradiol. In contrast to controls, ascending infection resulting in endometritis was found in animals receiving doses of greater than or equal to 1.0 microgram of estradiol per day. Response to estradiol treatment was reflected in an increase in cervical-uterine wet weight and uterine wall thickness. No differences were observed in time of appearance of antibody titers to GPIC in serum, but a delay in appearance of IgA antibody to GPIC in genital secretions was found in estradiol-treated animals receiving doses of greater than or equal to 1.0 microgram per day.