An animal model of chronic hypoplastic marrow failure (CHMF, aplastic anaemia), produced in mice by busulphan, was studied for changes in function and number of tissue macrophage populations. In vitro study of peritoneal macrophages showed no abnormality of function as assessed by chemotaxis, spreading, phagocytosis or bacterial killing. In vivo study of Kupffer cells showed a normal total number and a normal rate of clearance of intravenously injected colloidal carbon. Kupffer cells showed a reduced turnover rate as assessed by a cytoplasmic double-labelling technique and a reduced rate of carbon clearance following stimulation with endotoxin. The numbers of splenic and peritoneal macrophages were both to be decreased and the peritoneal macrophages also showed a decreased recruitment in response to intraperitoneal injection of casein. This decreased recruitment was corrected following marrow transplantation. These observations suggest that in experimental CHMF different tissue macrophage populations are variably decreased in number and that the recruitment of new macrophages is decreased, particularly in response to stimulation. The observations on peritoneal macrophages suggest that at least some of the changes can be directly related to the lesion of marrow precursors.