The metabolic activity of bone marrow cell suspensions was studied by the modified Lajtha method on samples from normal subjects, from nondialyzed and dialyzed uremics and from patients with hypoplastic anemia. It was found that: (1) the metabolic activity of bone marrow cells of the uremic nondialyzed patients is reduced as compared with the controls, and (2) the metabolic activity was reduced when marrow cells from normal subjects were incubated with creatinine, with methylguamdine or with uremic serum. These findings suggest that creatinine and methylguamdine play an important role in disturbing the cellular metabolism in uremia and they confirm that the anemia of chronic renal failure is largely due to metabolites which interfere in the maturation and proliferation of the cells.