In this article, I have attempted to summarize many studies that clearly demonstrate a major role of combinations of growth factors in the development of blood cells. Clearly, the most striking effects are seen on primitive cells that do not reach full proliferative potential when stimulated by single factors. The treatment of patients with combination growth factor therapy has high potential in the areas of chemotherapy and bone marrow transplantation. In these patients, more rapid regeneration of primitive cells will lead to more rapid and effective recovery. Also, combinations of factors are essential for optimal retrovirus-mediated gene transfer in repopulating stem cells for use in gene replacement therapy. Stem cell factor has been shown to protect burst-forming unitserythroid from the toxic effects of zidovudine (AZT). This is an example where combinations of factors may enable patients to continue treatment regimens for extended periods. The side effects resulting from some factors may be overcome using combination therapy. When combinations of factors are given, greatly reduced doses of each factor can produce results similar to those seen with high doses of a single factor.