Sixteen infants with the anemia of prematurity were randomly assigned to treatment with packed erythrocyte transfusions, recombinant erythropoietin alone, or epo plus recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF). During the treatment period, blood reticulocyte concentrations remained unchanged in those randomly assigned to receive transfusions, but increased significantly in those who received erythropoietin, either alone or in combination with GM-CSF. The magnitude of increase in hematocrit was not greater in those who received erythropoietin plus GM-CSF than in those who received erythropoietin alone. Blood neutrophil concentrations fell in all four infants who received erythropoietin alone, but increased in all who received erythropoietin plus GM-CSF.