In order to study the megakaryocyte function in vitamin C-deficiency (avitaminosis) the author observed the fluctuation of the thrombocyte count in experimental vitamin C-deficient guinea pigs, and next, by performing the bone marrow tissue culture of the femur of these animals, the following results were obtained: 1. The thrombocyte count decreased markedly in the vitamin C-deficient guinea pigs. 2. The megakaryocyte function in these vitamin C-deficient animals was markedly lowered as compared with that of the normal. 3. The lowered megakaryocyte function in the vitamin C-deficient guinea pigs could be brought back to the normal level by adding vitamin C 0.01 mg/c.c. directly to the medium. From these findings it has been clarified that vitamin C and the function of megakaryocytes are closely related because the lowering of the magakaryocyte function is induced by the decrease in the number of thrombocytes in vitamin C-deficiency.