The purpose of this work was to determine platelet and myeloid cell-specific requirements for β3-containing integrins in hemostasis, bone resorption, and tumor growth. LoxP-flanked mice were generated to study the conditional deletion of β3-integrin in platelets [knockout in platelets (KOP)] and myeloid cells [knockout in myeloid (KOM)]. Using the β3KOP and β3KOM strains of mice, we studied the role of β3-integrin in hemostasis, bone resorption, and subcutaneous tumor growth. Tissue-specific deletion of platelet β3-integrins in β3KOP mice did not affect bone mass but resulted in a severe bleeding phenotype. No growth difference of tumor xenografts or in neoangiogenesis were found in β3KOP mice, in contrast to the defects observed in germline β3−/− mice. Conditional deletion of myeloid β3-integrins in β3KOM mice resulted in osteopetrosis but had no effect on hemostasis or mortality. Tumor growth in β3KOM mice was increased and accompanied by decreased macrophage infiltration, without increase in blood vessel number. Platelet β3-integrin deficiency was sufficient to disrupt hemostasis but had no effect on bone mass or tumor growth. Myeloid-specific β3-integrin deletion was sufficient to perturb bone mass and enhance tumor growth due to reduced macrophage infiltration in the tumors. These results suggest that β3-integrins have cell-specific roles in complex biological processes.—Morgan, EA, Schneider, JG, Baroni, TE, Uluçkan, Ö., Heller, E., Hurchla, MA, Deng, H., Floyd, D., Berdy, A., Prior, JL, Piwnica-Worms, D., Teitelbaum, SL, Ross, FP, Weilbaecher, KN Dissection of platelet and myeloid cell defects by conditional targeting of the β3-integrin subunit.