Cyclosporin A (CsA) is an immunosuppressive agent that inhibits the synthesis of lymphd by T lymphocytes at the level of transcription. A cytoplasmic protein, cyclophilin, is the most thoroughly studied CsA-binding protein, but its ubiquitous presence in cells of all types raises questions about its role in immunsuppression. In an attempt to ascertain the presence of a cell surface receptor, we synthesized two polyvalent macromolecular CsA derivatives, CsA-BBa-ovalbumin and CsA-BBa-aminodextran (CBD), from the product of the photochemical reaction ofCsA and 4-benzoylbenzoic acid (CsA-BBa).