We have investigated the expression of interleukin-4 receptors (IL-4R) in acquired immunodeficiency syndrome (AIDS)-related Kaposi's Sarcoma (KS) in situ by immunohistochemistry. Frozen and fixed sections from five patch stage and two nodular stage KS lesions were stained with anti-IL-4R monoclonal antibody with similar results. Skin biopsies from the clinically apparent lesions and adjacent clinically uninvolved skin were also examined. We observed that individual KS cells lining the irregular vascular spaces were stained with anti-IL-4R antibody, although the degree of staining was variable. The epithelioid and oval cells appear to stain more than the spindle cells in plaque stages or nodular lesions. The sections from nonclinically involved skin also contained a few cells with features of KS, singly or in clusters that also stained for IL-4R. Skin sections from four normal donors did not stain with IL-4R antibody except for hair follicles, sweat glands, and faint staining of blood vessels. KS sections were also stained with antibodies to basic fibroblast growth factor (FGF), S100, fibronectin, and von Willebrand factor. KS lesions from clinically involved and uninvolved skin sections were positive for all four antibodies. Thus, the differences between KS lesion and clinically uninvolved skin adjacent to a KS lesion may be more quantitative than qualitative. The IL-4 receptors on KS cells were functional as IL-4 modulated intercellular adhesion molecule 1 (ICAM-1) on these cells. Taken together, our results suggest that AIDS-KS cells express elevated levels of IL-4R compared to normal endothelial and skin cells and, thus, the receptors for IL-4 on KS may serve as an attractive target for anticancer therapy.