The previous study showed that an unusual mineralized tissue appeared in the bone marrow of rats 5 days after colchicine injection. The bone marrow stroma cells isolated from the colchicine treated rats were cultured and characterized to elucidate the relationship between the bone marrow stroma cells and the unusual mineralized tissue. The bone marrow cells were obtained 1 to 5 days after the injection of colchicine and the attached cells (marrow stroma cells) were cultured and tested for alkaline phosphatase (ALP) activity as a marker of osteoblastic characteristics. The cultured cells isolated 2 days after the injection (Col. 2 cells) showed a low ALP activity during the initial culture period. However, the activity began to increase and showed a high level from the 14th to the 30th day of culture. The other cell populations did not show an increase of ALP activity during the culture period. The ALP of the Col. 2 cells resembled the bone type enzyme by their biochemical characteristics. The Col. 2 cells became to respond to both the parathyroid hormone and prostaglandin E2 during the late stage of culture period. These findings suggest that the bone marrow stroma cells isolated from the rats 2 days after the colchicine injection become enriched with osteoblast precursor cells. These cells might differentiate into the osteoblasts during the repair process following the severe damage after the colchicine injection and produce an unusual mineralized tissue in the bone marrow cavity.