Another potential use of human CEACAM1 transgenic mice is to study pathogenesis where CEACAM1 functions as the pathogen receptor. The situation is especially critical in the case of Neisseria gonorrhoeae infections where there is a pressing need for the development of a vaccine [2] . Since the high expression of CEACAM1 on neutrophils plays an important role in its pathogenesis, we determined the expression levels of huCEA-CAM1 on neutrophils isolated from the bone marrow and blood of the transgenic animals. As shown in Fig. 7 , the expression of murine and human CEACAM1 in the transgenic animals was strong. As shown in Fig. 8(A-C) , the staining of murine and human CEACAM1 was strong for neutrophils in blood smears taken from the human CEACAM1+/2 transgenic animals, while staining for human CEACAM1 was negative for a CEACAM12/ 2 littermate. For purposes of comparison, neutrophils from a human blood smear were stained (Fig. 8D) . Notably, human neutrophils stain more intensely than those from the transgenic animals, suggesting a possible effect of an expression difference between the species. In addition, we examined the expression of human CEACAM1 in the spleen of transgenic animals. Based on cell surface staining, there was no expression on T-cells and significant expression on B-cells (Fig. 9A) . Consistent with these results, CEACAM1 has been shown to be inducible on both murine [45] and human T-cells [14] . The expression of CEACAM1 on murine [46] and human [17, 47] B-cells has been previously reported. Activation of splenocytes from transgenic animals by anti-CD3/28 antibodies for three days revealed increased expression of murine, but not human CEACAM1 in T-cells (Fig. 9B) . B-cell expression of murine and human CEACAM1 is shown for comparison.