A truncated form of the full-length, immunosuppression-inducing molecule, CD200, CD200tr, was expressed in CHO cells and the transduced cells used to produce mAbs to CD200tr. These mAbs detect simultaneous expression of full-length CD200 and CD200tr in LPS-stimulated dendritic cells (DCs). Moreover, CHO cells and DCs expressing CD200tr as well as soluble CD200tr are able to inhibit suppression of mixed leucocyte reactions in vitro which follows addition of the soluble form of CD200 (CD200:Fc) in culture, demonstrating that CD200tr is a physiological antagonist of CD200.