The human melanoma growth-stimulatory activities (MGSA alpha, beta, gamma/GRO) are products of immediate early genes coding for cytokines that exhibit sequence similarity to platelet factor-4 and beta-thromboglobulin. MGSA/GRO alpha has been demonstrated to partially complete for binding to the approximately 58-kDa neutrophil receptor for another beta-thromboglobulin-related chemotactic protein, IL-8. We demonstrate that when [125I]MGSA/GRO alpha was cross-linked to receptors/binding proteins from human placenta, there were two major [125I]MGSA cross-linked bands of approximately 64,000 and approximately 84,000 Mr. Because [125I]MGSA exists primarily in monomer and dimer forms at the concentrations used here, it is not clear whether the receptor/binding proteins represented by the cross-linked bands are approximately 50,000 and approximately 70,000 or approximately 58,000 and approximately 78,000 Mr. Ligand binding to the receptor proteins is associated with enhanced tyrosine phosphorylation of a number of substrates, including proteins in the same Mr range as the MGSA/GRO receptor/binding proteins.