Beta-adrenergic receptors on cultured neurofibromatosis (NF) skin and tumor fibroblasts were characterized by a radioligand binding assay using (-)-125I-iodocyanopindolol (ICYP), a potent beta-antagonist. NF tumor fibroblasts had a very low density of beta-adrenergic receptors (0-450 binding sites/cell) when compared to fibroblasts obtained from a healthy skin area of NF patients or from healthy controls (average 1,800 binding sites/cell). The apparently low binding of ICYP to the NF tumor fibroblasts can be an expression of an alteration in the structure or function of the plasma membrane.