Using a whole-cell antigen antibody to Trichomonas vaginalis was measured by an enzyme-linked immunosorbent assay (ELISA). IgG antibody was found in sera from only three of 99 children under 12 years of age. In contrast, serum IgG or IgM antibody or both were detected in 80.4% of women who had vaginal trichomoniasis and in 13.7% of uninfected women. Although antibody was found in cervical and vaginal secretions, the correlation between current infection and the presence of antibody was poorer than found between circulating antibody and infection. IgG or IgA antibody or both was detected in the secretions of 73.2% and 41% of infected and uninfected women respectively. This may be accounted for, at least partly, by previous infections since antibody, circulating or local, was found most often in women who had a history of trichomoniasis. There was no indication that some other vaginal micro-organism stimulated antibody directed against T vaginalis.