Mouse anti-rat OX-2 monoclonal antibody recognizes a monomorphic determinant present on rat thymocytes, brain, follicular dendritic cells in lymphoid organs, vascular endothelium, and at low levels on some smooth muscle and B lymphocytes. The purified brain and thymocyte OX-2 antigens are glycoproteins with apparent M.W. of 41 KDa and 47 KDa respectively. The amino acid composition of brain and thymocyte OX-2 antigen are very similar and are antigenically similar to those found on other tissues. The carbohydrate composition shows that this antigen is highly glycosylated (brain OX-2 - 24% and thymocyte OX-2 - 33% carbohydrate by weight). The OX-2 antigen shows similarities to the Thy-1 antigen in its odd pattern of tissue distribution, carbohydrate composition and characteristic migration on SDS- PAGE. Also, the OX-2 antigens, like Thy-1 antigens, have homologies with immunoglobulin domains the overall structure of OX-2 is similar to an Ig light chain or the T cell receptor chain. Because of its distribution, it is thought to play a role in mediating recognition events at cell surfaces. This clone can be used in flow cytometry, affinity chromatography, immunohistochemistry and in binding assays. It is also useful for labeling the follicular dendritic cells thought to be involved in the generation of B cell memory as it does not label the Ia-positive dendritic cells present in the T-dependant areas of lymphoid organs.