The hematopoietic function of the bone marrow of renal patients and influences of azotemia on the normal bone marrow were studied by means of clinical bone marrow tissue culture. The results obtained are as follows. 1) The marrow myelopoietic function of nephrotic patients was increased, when normal human serum was added to the medium. 2) The myelopoietic function in acute glomerulonephritis showed no significant change in regard to the cell density index but the relative growth rate and cellular migratory velocity were slightly decreased, when the bone marrow was cultured in a medium containing normal human serum. 3) When the bone marrow from patients with chronic glomerulonephritis was cultured in a medium containing normal human serum, the marrow function became activated in a slightly elevated NPN group while it became dep. essed in a highly elevated NPN group. 4) The bone marrow from uremic patients cultured in a medium containing normal human serum showed a growth pattern simulating that of hypoplastic anemia. The relative growth rate was half the control value and the migratory velocity of neutrophilic cells and cell density index were markedly decreased. 5) The bone marrow from patients with chronic renal insufficiency was cultured, to investigate the megakaryocytic function, in a medium containing normal human serum. There was reduction in both megakaryocyte number and percentage of motile forms of megakaryocytes appearing in the growth zone. 6) The normal bone marrow showed a reduced relative growth rate and migratory velocity of neutrophilic cells, when cultured in a medium containing azotemic serum. 7) Similarly, the megakaryocytic function became depressed by an addition of azotemic serum to the medium. From the evifence obtained, it appears that azotemia accompanying chronic renal insufficiency exerts a depressing action on both myelopoietic and megakaryocytic functions. It is conceivable that the bone marrow under such an inhibitory action of a long standing will gradually show a transition to a hypoplastic state.