The reactivity of the antibovine monoclonal antibodies (mAbs) comprising temporary cluster TC1 was compared with that of two OvCD45R mAbs on sheep cells. Three of the mAbs-CC31, CC99 and CC103-did not cross-react with sheep cells. All the workshop mAbs precipitated two molecules of apparent molecular weight (MW) 200 kDa and 220 kDa while the antisheep CD45R mAb 20-96 precipitated a single band of 220 kDa. Cell surface expression was examined by single colour FACS (fluorescence activated cell sorting) analysis of efferent and afferent lymph cells and peripheral blood lymphocytes and the distribution of the antigens on CD4+, CD8+ and T19+ (WC1) and B cells was determined by two colour fluorescence staining. By cellular distribution and immunohistology the TCl mAbs could be divided into four distinct groups which differed from a fifth group comprising the two OvCD45R antibodies.