Two types of adipose cells were found in the connective tissue on day 7 after bone matrix gelatin (BMG) implantation and an injection of bupivacaine mature adipose cells with a large lipid droplet (2-140 µm) and immature adipos cells with many small lipid droplets (0.1-2 µm). On day 10 after BMG implanta tion, typical adipose tissue was observed near the implant. The immature adi pose cells had small, spherical mitochondria, glycogen granules and cytoplasm!; microvesicles, and they might differentiate from undifferentiated mesenchym;ι cells in the connective tissue or the peripheral cells around the vessels as a whiu adipose tissue. These findings suggest that the differentiation of adipose cells in the connective tissue near heterotopic bone formation might be induced no only by mechanical and/or bupivacaine injury, but also by some factor or factor of the B