Recent progress in molecular cloning has provided access to several major human colony-stimulating factors - GM-CSF, IL-3, G-CSF and M-CSF. Now they are available highly purified from different expression systems (e.g. yeast, E. coli, CHO cells). These molecules, acting multifunctionally in the hematopoietic system, are responsible for proliferation and differentiation of bone marrow-derived progenitor cells in vitro. First clinical studies performed with colony-stimulating factors have shown that neutropenia caused by drug-induced immunosuppression in patients with refractory cancer or autologous bone marrow transplantation was reversed using rh GM-CSF or rh G-CSF. We have investigated the effect of the consecutive administration of rh IL-3 and GM-CSF on hematopoiesis in normal cynomolgus monkeys. Whereas administration of rh IL-3 alone did not result in an increase of WBC counts, the combination therapy of rh IL-3 followed by rh GM-CSF exhibited significant synergistic effects and raised WBC numbers. Furthermore, application of both factors resulted in a platelet rise not seen when one of the factors was used alone. The response was dose dependent and implicated that the therapeutical potential of rh GM-CSF could be expanded if used in combination with rh IL-3.