To investigate the importance of chlamydiae, ureaplasmas, Mycoplasma hominis, and anaerobic bacteria in the pathogenesis of neonatal conjunctivitis in the Harrow population conjunctival specimens from 104 infants with conjunctivitis and 104 similar healthy neonates were examined. The incidence of neonatal conjunctivitis was 8-2%, and no case of neomycin-resistant disease occurred during the study. Staphylococcus aureus, viridans Streptococci, and Escherichia coli were the only micro-organisms isolated significantly more frequently from affected than from control eyes, which suggests that these bacteria may be a cause of the conjunctivitis. All cultures for chlamydiae, M. hominis, Neisseria gonorrhoeae, and anaerobic bacteria were negative. The mother's race, social status, illness, and obstetric events were found to have no effect on the incidence, time of onset of conjunctivitis, or micro-organisms isolated. The clinical characteristics of conjunctivitis were also not related to the micro-organisms isolated. No potential pathogens were isolated from 63-5% of the eyes showing conjunctivitis. The results suggest that some of these cases may be caused by chemical irritation, and the possibility of an infectious aetiology is also discussed.