Bacterially synthesised human granulocyte colony-stimulating factor (G-CSF) was administered to patients with advanced cancer. The immediate effect of G-CSF was a fall in the level of circulating neutrophils followed by a rise after 4 hours that was sustained during G-CSF administration. The rise in neutrophil level was less in patients who had been treated previously with chemotherapy and/or radiotherapy. G-CSF was also administered to patients following melphalan and this resulted in a reduction in the duration of the neutropenia that invariably follows melphalan. G-CSF was well tolerated and did not have to be stopped in any patient.