Maternal chlamydial antibodies were determined in cord sera of 41 infants who developed neonatal chlamydial conjunctivitis and compared with the antibody profile of infants who had been exposed to Chlamydia trachomatis at birth by their isolation positive mothers but in whom conjunctivitis did not develop. No protective effect could be attributed to maternal antibodies transferred to the infants. Paired sera samples were collected from 18 infants with chlamydial conjunctivitis. Chlamydial IgM antibodies were detected in four of these 18 cases at the time diagnosis was established by isolation. An additional eight cases had developed chlamydial IgM at the time the convalescent sera samples were taken, on average on day 40. At that time symptoms had disappeared after systemic treatment had been given. Thus chlamydial IgM antibodies were eventually shown in two thirds of infants with chlamydial conjunctivitis who were all systemically treated and clinically healed. These data suggest a cautious assessment of chlamydial IgM in the diagnosis of chlamydial pneumonia.