Whatever the major functions of the eosinophil—whether related to cytotoxicity, modulation of inflammatory responses, or some other aspect of host defense—it is clear that intimate relationships must be established between this cell and its surroundings. These relationships necessarily involve the eosinophil's surface so that an understanding of its features, particularly those features termed its “membrane receptors,” is essential for an appreciation of the range of this cell's functional potential. A number of recent investigations have disclosed the presence of immunoglobulin and complement receptors on eosinophils; however, there has been considerable variation in the findings. In the present study we have undertaken first to define some of the important quantitative and biologic features of these receptors which might account for the differences reported from various laboratories and then to relate our findings directly to specific cell-parasite interactions using a schistosomule-leukocyte adherence assay. Eosinophils from normal individuals and from patients with helminthic infections were studied and compared.