Cedarlane's anti-ratCD45 monoclonal antibody recognizes a monomorphic determinant of the ratleukocyte common antigen (1). The antigen recognized is a heavily glycosylatedmembrane glycoprotein of molecular weight 170,000 Da on thymocytes butmolecular weight 170,000-220,000 Da on other leukocytes. The leukocyte commonantigen (L-CA) is a major glycoprotein of haematopoietic cells but is not foundon other tissues or erythroid cells.