A new bovine B-cell differentiation antigen is described that is detected by three monoclonal antibodies (mAb). The antigen is not an immunoglobulin and is precipitated from peripheral B cells as a molecule with an approximate molecular weight (MW) of 120,000 or 145,000 before and after reduction, respectively. Data obtained from two-colour cytofluorimetry and immunohistochemistry confirmed that the antigen was found only on mature B cells and on cells with dendritic morphology in the follicles of the organized lymphoid tissues. Its level of expression is directly correlated with that of IgM on peripheral blood B cells and Theileria parva-transformed B cells. The marker was also expressed on the peripheral cells which expressed surface IgG. Based on the antigen's cellular distribution, biochemistry and histochemistry, it is considered to be analogous to the human CD21 antigen.