Granulocyte colony-stimulating factor stimulates the proliferation and differentiation of progenitor cells committed to the neutrophil lineage, and it has been shown to improve survival to bacterial challenge in neutropenic mice. We studied recombinant human granulocyte colony-stimulating factor (rhG-CSF), cloned from bladder cell carcinoma line 5637, in a nonneutropenic infection model of Streptococcus pneumoniae pulmonary infection in splenectomized and sham-operated control mice. The rhG-CSF improved survival in the splenectomized mice but not in the sham-operated mice. Circulating leukocyte counts were greatest for the rhG-CSF-treated splenectomized mice compared with all other groups, presumably due to a loss of splenic sequestration. Clearance of live pneumococci from mouse lung pairs was impaired after splenectomy. The rhG-CSF improved lung clearance in both splenectomized and sham-operated mice compared with saline solution-treated controls. The number of live pneumococci recovered from tracheobronchial lymph nodes at 24 hours after aerosol challenge was greatest in the splenectomized mice vs sham-operated mice. Decreased numbers of viable pneumococci were recovered from tracheobronchial lymph nodes from the rhG-CSF-treated splenectomized mice and the sham-operated mice vs saline solution-treated controls. The rhG-CSF may be a useful adjuvant for treating infections in individuals with immunologic dysfunctions other than neutropenia.