The reactivity of sera from 151 confirmed human cases of Capillaria philippinensis infection was examined by double diffusion and indirect hemagglutination (IHA) tests chiefly against Capillaria obsignata antigen because of the present unavailability of C. philippinensis antigen. Antigens from additional parasites and other human and animal sera representing a variety of helminthic infections were used for comparison. Of 71 pre-treatment human sera, 56.3% were reactive by double diffusion test and 85.9% by IHA test (titer >1:16) with C. obsignata antigen. C. philippinensis sera were also reactive with Trichinella spiralis and Trichuris vulpis antigens but not with Schistosoma japonicum antigens. Sera from other infections such as with T. spiralis, T. vulpis, and S. japonicum were also reactive with C. obsignata antigen but sera from Trichuris trichiura infection were not. With C. obsignata antigen, IHA titers in human C. philippinensis sera are apparently not related to clinical severity of the disease; the titers remain at fairly stable levels during the course of the illness but may tend to decrease after chemotherapy. The cross-reactivities observed dictate caution in the use and interpretation of any serologic procedure for human intestinal capillariasis; nevertheless, the IHA test using C. obsignata antigen may be a useful additional tool in the study of C. philippinensis infection both for clinical and epidemiologic purposes especially when the efficiency of stool examination is decreased by changes in the reproductive activity of the helminth.