By carrying out a series of observations on the protein metabolism by bone marrow culture (Carrel flask method) and on the effect of various drugs acting directly on the bone marrow, the following results have been obtained: (1) Albumin is utilized for multiplication of the bone marrow tissue while globulin is produced by the bone marrow. Consequently, the increase of globulin has a correlation with marrow functions in direct proportion. (2) In globulin fractions, γ-globulin seems to play the most important rôle. (3) Both α- and β-globulins seem to play no important rôle. (4) Fibrinogen is produced by the bone marrow, and it has a close relationship with marrow functions. (5) Folic acid, vitamin B12, and bone marrow extract directly stimulate marrow functions, and they promote the production of globulin, especially of γ-globulin and fibrinogen on one hand, and the consumption of albumin on the other hand. (6) Nitromin (Nitrogen mustard-N-oxyde) lowers functions of the bone marrow and inhibits the production of globulin, paricularly γ-globulin and fibrinogen, and the consumption of albumin.