The purpose of this investigation was to examine the effects of medium type (α-MEM or DMEM); serum amount (5, 10 or 15%); and dexamethasone (Dex) induction (Dex (-) or Dex (+)) on the proliferation and alkaline phosphatase (ALP) activity of twice-passaged SD rat's bone marrow stromal cells. (1) After 6 days culture (before confluence), cell number and total protein concentration were measured. It was found that the serum amount played the dominant role in increasing the proliferation. (2) After 24 days culture (long after confluence), total protein concentration and ALP activity were measured. It was found that α-MEM was superior to DMEM in producing larger total protein concen-trations. Incrementing the serum amount tended to the increase in total protein concen-trations. Dex induction (Dex(+)) decreased values of ALP activity / total protein concen-tration, reflecting the advance of the osteogenic differentiation stage. The use of Dex (+)α-MEM medium was preferred to produce greater protein production with higher os-teogenic differentiation level