An in vitro assay for the new lymphokine eosinophil stimulation promoter has been adapted for use with human material. Peripheral eosinophils from patients with schistosomiasis mansoni were specifically induced to migrate on incubation with egg antigen. Furthermore, the peripheral lymphocytes of these patients on incubation with the egg antigen secreted the lymphokine eosinophil stimulation promoter, which enhanced the migration of purified eosinophils from patients with or without schistosomiasis. The test can be easily performed with human target cells and may be helpful for diagnostic or investigative purposes.