We were especially interested in the expression of the human gene in the immune system. Murine CEACAM1 is known to be expressed on spleen B-cells [50] , serving as a receptor for the murine hepatitis virus. Similarly, human CEACAM1 is expressed on germinal center B-cells [17] . In the case of murine and human T-cells, little or no surface expression is observed on resting Tcells, but is highly expressed on activated T-cells [14, 51] . As expected, we found cell surface expression of both murine and human CEACAM1 on splenic B-cells, but not on splenic T-cells. Similar expression of murine and human CEACAM1 is expected on the surface of neutrophils, whether from the bone marrow or blood. Indeed, we found high levels of murine and human CEACAM1 on neutrophils from either source. Based on this preliminary analysis, it is likely that the CEACAM1 expression in the immune system mimics that found in mouse and man.