Circadian coordination of the components of the hematopoietic system, immune system, and host-tumor balance in animals and humans have been demonstrated. Preclinical studies provide evidence that when within-the-day exogenous protein growth modulators such as EPO, G-CSF, IL-1, IL-2, TNF, or IFN are administered to animals, they determine to a large extent the magnitude of the hematopoietic, immunologic, toxic, and antitumor response. Optimal circadian timing of therapeutic proteins with minimal toxicity (e.g. EPO, CSF) should be further explored to investigate whether lower doses could be used or intervals prolonged with equal or greater drug efficacy. For antitumor efficacy, optimal circadian timing of growth factors may lower host toxicity, which may allow higher doses to be more safely utilized to more reliably induce significant anticancer activity.