The colony-stimulating factors are a group of growth factors important in regulating the production of myeloid cells. The past 25 years have seen the identification and characterization of many of these growth factors and, more recently, the molecular cloning of their genes. This has enabled the production of sufficient quantities to assess their biological activity in vivo and in vitro. Some of these recombinant growth factors have also been employed in clinical trials, which have indicated potential uses in the treatment of a variety of diseases. Here, Anthony Whetton considers the biology of haematopoietic growth factors, and the evidence that they may be of value in the treatment of haematopoietic, infectious and malignant disease.