1. Hematopoietic actions of cobalt and copper, either singly or in combination with iron have been studied by means of rabbit bone marrow culture in fluid medium. 2. When various cobalt compounds are added singly, with exception of cobalt chloride, every one of them acts markedly to help erythropoiesis, but Hb content on the contrary often decreases by such addition. Consequently, this hypochrome erythropoiesis seems to be due to lack of iron supply. 3. In the case where ferric gluconate is added in combination with cobalt compounds, Hb increses markedly, especially so when the combination is ferric gluconate and cobalt chloride in proportion of 1:50; suggesting that cobalt greatly assists the induction of iron into the heme nucleus. 4. Due to its toxicity, copper addition either singly or in combination with iron does not help erythropoiesis to any great extent, but when in combination with iron as in the case of cobalt copper helps to increase Hb content. 5. The addition of three metals, iron, copper, and cobalt, does not yield the best result, indicating the necessity of giving a consideration to the toxicity arising out of the amount to added
