The sensitivity specificity of an enzyme-linked immunosorbent assay (ELISA) for the serodiagnosis of strongyloidiasis has been investigated. 45 men with long-standing strongyloidiasis were compared with the same number of age- and sex-matched control subjects. The ELISA detected antibody in 84% of patients with parasitologically proven strongyloidiasis. When the technique was compared with an indirect immunofluorescent assay (IFA), a high correlation coefficient was obtained. Specificity was demonstrated by observing a marked fall in optical density of pooled positive serum after prior incubation with Strongyloides ratti soluble antigen but not after incubation with antigens derived from Ascaris suum or Dirofilaria immitis. The test is simple and offers a useful method for the diagnosis of strongyloidiasis. In these patients it was more reliable than a single parasitological examination of faeces or duodenal contents.