Most studies that use recombinant granulocytopoietic cytokines, such as granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), with the intent of attenuating neutropenia generally have delayed the administration of the cytokine until 24 to 72 hours following completion of chemotherapy. This practice was initiated out of theoretic concern that colony-stimulating factor administration may cycle and differentiate a population of normal cells, thus increasing their susceptibility to cycle-specific antineoplastic agents. The theory, in fact, has been substantiated by evidence from several clinical trials of concurrent administration. Thus, simultaneous administration of chemotherapy and G-CSF or GM-CSF should be limited to investigational protocols with scientific objectives, such as cycle compression or malignant cell sensitization.