An enzyme-linked immunosorbent assay-(ELISA) using whole Trichomonas vaginalis as antigen was developed for measurement of serum antibody to T. vaginalis. Sera from six women who denied ever having had genital contact were used as the negative control. Of 38 women with proved T. vaginalis infection, 25 (66%) had elevated ELISA values. Values were usually very stable over weeks to months of follow-up. Among a matched comparison group of 38 women attending the same clinic who did not have T. vaginalis infection (as detected by wet mounts), an elevated value was present in only eight (21%) of 38 (P less than 0.001). Thus, in this group of women, the sensitivity was 66%, the specificity, 79%, the predictive value of a positive test, 76%, and the predictive value of a negative test, 70%. Our ELISA clearly demonstrates more reactivity in women with T. vaginalis. Its usefulness as a marker of current infection is probably limited, but it could be of considerable value for seroepidemiologic studies.