A 27 year-old woman presented with disseminated infection due to Mycobacterium kansasii. Signs and symptoms of disseminated infection persisted despite the administration of multiple antimycobacterial agents to which her organism was sensitive for 15 months. She was seronegative for HIV-1 and functional studies of T and B lymphocytes and granulocytes failed to demonstrate any abnormality. Peripheral blood monocytes proved abnormally permissive to the intracellular growth of Mycobacterium avium and M. kansasii, and expressed normal number of receptors to interferon-gamma, but reduced numbers of receptors to granulocyte monocyte colony stimulating factor and tumor necrosis factor. These defects were partially reversed with in vitro exposure of her cells to recombinant GM-CSF. In addition, administration of recombinant human GM-CSF in vivo (250 mg/M2 per day) for 10 days armed her circulating monocytes as evidenced by increased production of O2- in response to phorbol esther and, when infected ex vivo with M. kansasii, enhanced inhibition of intracellular growth compared with pre-therapy monocytes. These defects reappeared with discontinuation of GM-CSF and resolved with its re-administration. While a salutary clinical and microbiologic effect was difficult to assess, administration of GM-CSF in vivo was associated with in vitro activation of monocytes and enhanced mycobactericidal activity in this patient with a defect in monocyte function.