The adult mouse spleen retains granulocytic progenitor cells that are capable of proliferation and maturation. In mice given cyclophosphamide, spleen weight and cell content increased. In vitro granulocyte progenitor cells increased more than 300-fold in the spleen and remained elevated 14 days following cyclophosphamide. Proliferative and nonproliferative granulocytes were increased above controls until Days 21 and 28, respectively. In splenectomized mice, blood neutrophil recovery was delayed when compared to nonsplenectomized mice following cyclophosphamide. Although marrow cell number was similar in both groups, maximum marrow granulocyte progenitor cells were 2-fold greater in nonsplenectomized animals. These studies demonstrate that the murine spleen becomes a markedly granulopoietic organ and is a major contributor to recovery of granulocytes following cyclophosphamide. Splenic granulopoiesis should be taken into account in studies which measure toxicity of various agents in murine hematopoiesis.