Sodium butyrate increased alkaline phosphatase (ALP) activity of cloned osteoblastic cell line MC3T3-E1 by the stimulation of de novo enzyme synthesis. 2. Sodium butyrate did not affect mature osteoblastic cells but affected preosteoblastic cells. 3. Sodium butyrate decreased tartrate resistant acid phosphatase (TRACP)-positive multinucleated cells (MNC) formation from bone marrow cells. This related to the cytotoxicity of sodium butyrate on bone marrow cells.