A test for the lymphokine eosinophil stimulation promoter (ESP) has been adapted to the study of Trichinella spiralis infections in mice and has been used as an aid in the diagnosis of trichinosis in a patient. With use of soluble antigen extracted from T. spiralis larvae and peritoneal exudate cells rich in eosinophils obtained from mice with trichinosis of four weeks' duration, a dose-response curve was constructed and analyzed. The soluble larval antigens greatly enhanced the migration of eosinophils from the mice with trichinosis but had no effect on the eosinophils from mice with schistosomiasis, nor did soluble schistosome egg antigens have any effect on the cells from mice with trichinosis. On two occasions peripheral white blood cells from a patient with presumed T. spiralis infection gave strongly positive results in ESP tests on stimulation with soluble larval antigen, but the cells did not respond significantly to soluble schistosome egg antigen; the result in the ESP test was positive before the patient's serum produced a weakly positive agglutination response.