Duplicate specimens were taken with cotton-wool swabs from the upper tarsal conjunctiva of 63 patients living in Gambian villages in which trachoma is endemic and from 34 infants with ophthalmia neonatorum (ON) attending an outpatient clinic in The Gambia. The detection of Chlamydia trachomatis by direct immunofluorescence (IF) using a conjugated monoclonal antibody to its principal outer membrane protein was compared with isolation in cycloheximide-treated McCoy cells. For trachoma, the sensitivity and specificity of the immunofluorescent technique were 62% and 100% respectively if ten elementary bodies (EBs) was taken as the minimum requirement for positivity by IF. If all cases with one or more EB were considered positive, the sensitivity was 81% and the specificity 85%. For ON the sensitivity and specificity were 100% and 95% respectively, regardless of which criterion was used. In view of its simplicity and easy applicability to field conditions it seems likely that direct IF using monoclonal antibodies may be a useful technique for the detection of C. trachomatis in the conjunctival epithelium of patients with trachoma.