The purpose of this study was to evaluate the effect of recombinant granulocyte‐macrophage colony‐stimulating factor (rGM‐CSF) on BALB/c mice infected s.c. with the intracellular pathogen Leishmania major. Daily i.p. application of 1 μ rGM‐CSF for 21 days following the infection led to an aggravated course of the disease in most animals. In no case was a therapeutic effect observed. In vitro analysis revealed that the parasite burden was approx. 2‐ to 7‐fold higher in the infected lesions, in the lymph nodes draining the infection and in the spleens of rGM‐CSF‐treated animals than in tissues from nontreated mice. L. major ‐infected macrophages obtained from chronically infected mice proliferated in the presence of rGM‐CSF in vitro without gaining antiparasitic effector function. However, antiparasitic effector function increased and macrophage growth was inhibited in the presence of recombinant interferon‐γ (IFN‐γ). These data indicate that rGM‐CSF‐induced macrophage proliferation alone is not sufficient to overcome infections with intracellular pathogens like L. major , since simultaneous activation of macrophages by IFN‐γ is required.