Since excretion of Strongyloides stercoralis larvae in infected individuals is often scanty and irregular, diagnosis by direct fecal examination is unreliable and difficult. Therefore, we have continued to define circumstances in which serodiagnosis is useful and to explore other methods, such as the immediate hypersensitivity skin test. Further experience with the ELISA test for antibody indicates that levels decline and often become negative 1 to 2 years after successful treatment. A collaborative study of World War II U.S. prisoners of war in the Far East indicates that a substantial number of them are still infected with S. stercoralis. Antibody to the parasite was found in 26 percent of 180 veterans, many of whom had elevated eosinophil levels. Early results with a skin test, using antigens prepared from larvae of S. stercoralis and S. ratti, have shown positive immediate reactions in all parasitologically proven strongyloidiasis cases. Additional experience is needed to define specificity and possible cross reactions in patients infected with other parasites. Monoclonal antibodies have been developed in mice immunized with somatic S. stercoralis antigen. Of the 6 reactive clones, 3 also cross-react with filarial whole worm antigen, but the other 3 appear to have greater specificity.