The survival, differentiation, proliferation and development of haemopoietic precursor cells and the functional activity of mature blood cells are all influenced by colony stimulating factors (CSFs). As haemopoietic cells rapidly die in the absence of appropriate CSF, the promotion of cell survival mediated by CSFs, or growth factors, is fundamental to all the other effects exerted by these factors. This enhancement of cell survival is distinct from the stimulation of proliferation. Here we show that the death of haemopoietic precursor cells on withdrawal of the relevant CSF. is due to active cell death, or apoptosis, indicating that CSFs promote cell survival by suppression of the process of apoptosis. The existence of a positive control mechanism regulating precursor cell survival has important implications both for the regulation of normal haemopoiesis and for tumorigenesis.