Monoclonal antibodies (mAbs) submitted to the workshop B cell panel were screened for reactivity with bovine surface immunoglobulin positive (SIg+) cells from tonsil, mesenteric lymph nodes (MLN), ileal Peyer's patches (IPP), thymus and peripheral blood by fluorescence activated cell sorter (FACS) analysis. All mAbs within B cell panel reacted with B cells except the negative control mAb VPM61. Although most mAbs stained the majority of B cells from different tissues, 14 mAbs did not stain B cells from IPP. Detailed FACS analysis of the 24 mAbs in preliminary clusters PC17, PC28, PC29, PC30, PC35, PC41 and PC43 showed all preliminary clusters (PC) to contain mAbs with similar and variable specificities. This absence of clear-cut clusters therefore did not permit a simple classification of B cell surface antigens via the B cell mAbs and suggests considerable complexity and variability of the B cell surface.