Human recombinant granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was administered to 14 patients with refractory aplastic anemia (AA). The effect of rhGM-CSF therapy on the lymphocyte phenotype; on the proliferative responses to the mitogen phytohemagglutinin, Candida albicans, and tetanus toxoid antigens; and on the natural killer (NK) activity of the circulating lymphocytes was studied. Samples were collected before (baseline) and twice during the rhGM-CSF administration. The absolute number of circulating lymphocytes remained relatively constant during the first period, but experienced a significant increase (P less than .001) during the second period. The increase was most prominent in the B cells (P less than .001), but the T cells (P less than .016) also increased. Detailed investigation of lymphocyte subsets showed an increase of the markers CD38 (Leu17), HLA-DR, and the transferrin receptor throughout the treatment course giving evidence of lymphoid cell activation. The NK cell activity was suppressed (P less than .008) throughout the treatment. However, proliferative responses to phytohemagglutinin, Candida antigen, and tetanus toxoid were unaffected. Although the mechanism is not yet defined, GM-CSF does induce activation and increase in absolute lymphoid cell number, especially B cells, together with a decrease in NK cytotoxicity. The implication of these immune cell changes in relation to host resistance to microorganisms remains to be established.