The sensitivity and specificity of an indirect immunfluorescent antibody assay for the diagnosis of human strongyloidiasis has been investigated. Sera were obtained from 160 Australian ex-servicemen who had been prisoners-of-war in Southeast Asia during World War II, 44 of whom were proven parasitologically to have strongyloidiasis; these men did not have concurrent infections with other helminths. In addition, sera were collected from 44 age- and sex-matched Australians who were not thought to have been exposed to S. stercoralis, and from 44 Filipino subjects. Antibodies were measured by using living filariform S. ratti larvae as the source of antigen. The assay was highly sensitive; antibodies were found at a titer of 1:4 or greater in 98% of men with strongyloidiasis and in 2% of Australian control subjects. Fifteen percent of exposed ex-servicemen in whom parasites had not been found had antibody titers of 1:4 or more, and it is thought that they had cryptic infections. Incubation of pooled positive sera with soluble S. ratti antigen produced a marked fall in antibody titer, but no changes were seen after incubation with soluble Ascaris suum or Dirofilaria immitis antigens. It is thought that this indicates that the test is specific and that the 84% of Filipinos with antibody titers of 1:4 or greater had unsuspected strongyloidiasis. When antibody titers against S. ratti were compared with those obtained using living filariform S. stercoralis larvae, a high correlation was found (r = 0.89, P < 0.001). It is concluded that this assay provides a simple, safe, and specific method for the diagnosis of strongyloidiasis.