Objectives. GPIlb integrin expression has been found on platelets and megakaryocytes, and more recently on immature hematopoietic progenitors. We set out to investigate expression of GPIIb in other hematopoietic cell lineages and, having detected it on mast cells, aimed to determine what possible role it might perform. Methods. We have made use of cultured human and murine bone marrow mast cells (BMMC) in order to characterize the expression of GPIIb. Further, BMMC cultures from wild type and GPIIb deficient (gpIlb(-/-)) mice were used for comparison of the adhesive properties mediated by this receptor. Finally, peritoneal mast cells were analyzed from both wild type and (gpIIb(-/-)) mice. Results. We demonstrate expression of GPIIb on cultured BMMC. Using cells derived from mice homozygous for a null allele of gbIIb we show that the absence of GPIIb has no effect on mast cells with respect to a number of measures of cell growth and differentiation. However, loss of GPIlb on BMMC results in an increase in surface expression of aV integrin, the alternative partner of GPIIIa. Conclusion. The results in this study demonstrate that GPIIb is expressed in human and murine mast cells. A function for GPIIb on mast cells is suggested by the altered adhesion of gbIIb(-/-) BMMC to fibronectin- and vitronectin-coated surfaces. Moreover, comparison of mast cells from the peritoneal cavity of wild type and gbIlb(-/-) mice indicates that GPIIb could influence the in vivo differentiation or homing of tissue mast cells. (c) 2005 International Society for Experimental Hematology. Published by Elsevier Inc.