The multilineage hematopoietic effects of IL-3 appear to be most important for its clinical use comprising especially leucocyte and platelet responses. This was demonstrated to be dose dependent characterising doses of 250 to 500 micrograms/m2/day subcutaneously as hematopoietic effective and well tolerable. Since preclinical data suggest synergism between IL-3 and GM-CSF hematopoietic effects of their sequential administration were evaluated in 15 patients with preserved hematopoietic function. An enhanced stimulation of megakaryopoiesis combined with more pronounced stimulation of granulopoiesis than IL-3 alone could be demonstrated. The cytokine combination of IL-3 and GM-CSF was thus used during chemotherapy induced myelosuppression. Data indicates beyond the known amelioration of myelosuppression by GM-CSF additional chances to enhance platelet recovery which can be of important clinical impact.