CC chemokine receptor 1 (CCR1) is expressed in neutrophils, monocytes, lymphocytes, and eosinophils, and binds the leukocyte chemoattractant and hematopoiesis regulator macrophage inflammatory protein (MIP)-1α, as well as several related CC chemokines. Four other CCR subtypes are known; their leukocyte and chemokine specificities overlap with, but are not identical to, CCR1, suggesting that CCR1 has both redundant and specific biologic roles. To test this, we have developed CCR1-deficient mice (−/−) by targeted gene disruption. Although the distribution of mature leukocytes was normal, steady state and induced trafficking and proliferation of myeloid progenitor cells were disordered in −/− mice. Moreover, mature neutrophils from −/− mice failed to chemotax in vitro and failed to mobilize into peripheral blood in vivo in response to MIP-1α. Consistent with this, −/− mice had accelerated mortality when challenged with Aspergillus fumigatus, a fungus controlled principally by neutrophils. To test the role of CCR1 in granuloma formation, we injected Schistosoma mansoni eggs intravenously, and observed a 40% reduction in the size of lung granulomas in −/− mice compared to +/+ littermates. This was associated with increased interferon-γ and decreased interleukin-4 production in −/− versus +/+ lung lymph node cells stimulated with egg-specific antigen, suggesting that CCR1 influences the inflammatory response not only through direct effects on leukocyte chemotaxis, but also through effects on the type 1–type 2 cytokine balance. Thus CCR1 has nonredundant functions in hematopoiesis, host defense, and inflammation.