In bone marrow from 3 patients with prolonged, severe self-induced starvation, fat atrophy, hypoplasia of haemopoietic cells and characteristic gelatinous transformation of marrow were noted. The gelatinous substance appeared amorphous and stained pink with the Wright-Giemsa stains. Histochemical and ultrasturctural studies indicated that it consisted of acid mucopolysaccharides and was extracellular in nature. Similar marrow abnormalities were produced in rabbits by limitating their food intake for 4 months. These marrow abnormalities in the experimental animals could be reverted to normal by restoring their nutritional status. It is proposed that the gelatinous transformation of marrow is caused by excessive production of mucopolysacchrides of the ground substance to compensate for the mobilization of marrow fat which occurs to meet the energy requirement. It is further postulated that excessive production of acid mucopolysaccharides may provide a microenvironment unsuitable for haemopoietic proliferation. The relevance of these findings to other conditions associated with marrow aplasia, is discussed.