Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a haematopoietic growth factor with a wide variety of applications in the clinic. In early phase I studies the continuous intravenous (c.i.) route of administration was often used. Later it was shown that subcutaneous (s.c.) administration was also effective. The optimal route of administration remains, however, poorly defined, and no studies have made a direct comparison between these two routes of administration. We treated patients with advanced breast cancer with moderately high-dose doxorubicin and cylophosphamide and GM-CSF. The first 14 patients received GM-CSF by c.i, while subsequently 47 patients received it s.c. Comparison between the two groups showed that c.i. GM-CSF was more toxic in several respects. There was a higher need for erythrocyte and platelet transfusions and a significant deterioration in the performance status. This study indicates that subcutaneous GM-CSF is the preferred route of administration. Randomised trials are, however, needed to confirm these conclusions.