We sought to assess the antimicrobial capacity of human colostrum against Chlamydia trachomatis. a common agent of ophthalmia neonatorum. Colostrum was collected from 13 post-partum females and tested in an in vitro assay of chlamydial growth inhibition using HeLa 229 cells as the host cell line. All samples significantly inhibited chlamydial growth in a dose-response manner. The percent inhibition ranged from 45.3 to 99.0 (mean=88.1+/-4.1). The chlamydial growth inhibition activity of colostrum was found to be: heat- and freezing-resistant: more concentrated in colostrum than breast milk; was not attributable to interferon or antibody activity; and, could not be attributed to host cell cytotoxicity. Additionally, chlamydial growth inhibition occurred in < or = 15 min and was effective only when colostrum was incubated with chlamydiae prior to addition to HeLa 229 monolayers. Lastly, centrifugal fractionation of the colostrum yielded similar activity in the lipid pellicle and in the lipid-free supernatant. These results indicate that topically applied colostrum may have efficacy in the prophylaxis of ophthalmia neonatorum of chlamydial etiology in the absence of conventional modalities.