These studies were undertaken to determine if schistosomal and ragweed antigens incubated with human peripheral blood mononuclear cells (MNC) would stimulate the production of an eosinophil colony stimulating factor (CSF) which was active on human bone marrow. These studies have shown that schistosoma mansoni antigen in a concentration of 5 micrograms/ml incubated with MNC's from unsensitized individuals lead to production of a conditioned medium which did not stimulate an increased number of total colonies but did increase the number of eosinophil colonies (44 to 61 colonies). Ragweed antigen also stimulated increased eosinophil CSF (from 44 to 63 colonies). The granulocyte response to parasitic antigens almost invariably involves the eosinophil system. Eosinophils play a vital role in the control of these diseases in humans. Further understanding of the interaction of parasitic antigens with the granulopoietic system will hopefully give new and important clues as to how such parasites interact with man and how they are controlled.