The diagnostic roles of enzyme immunoassay and indirect immunofluorescence tests were studied in 99 patients with various grades of active trachoma and in 72 patients with inactive trachoma. Normal invididuals and patients with non-specific chronic conjunctivitis were used as a control group. Chlamydial antigens were detected in the conjunctival swabs from a large proportion of patients with severe or moderate trachoma. Cases with 'antigen-negative' but clinically severe trachoma showed presence of specific IgG in the blood, thus indicating that serological tests may complement the ELISA test for the detection of chlamydia in infected tissues. The nature of cellular reaction in the conjunctiva seems to be related to the severity of trachoma and presence of the infective organism. 'Antigen-negative' trachoma thus represents either an error of diagnosis or limited sensitivity of currently available immunologic techniques, or a predominantly hypersensitivity state triggered by transient tissue parasitism. Recombinant DNA technology and chronobiologic study of infiltrating lymphocytes are likely to provide some insight in such cases.