Osteoinductive activity of the transitional epithelium in the urinary bladder increased by more than 3 times in aging guinea pigs. The count of inducible osteogenic precursors and their sensitivity to urinary bladder osteoinductive factor decreased by several times with age. Our experiments show that proliferation of some stromal precursors in the spleen requires not only platelet-derived factors, but also growth factors produced by urinary bladder transitional epithelium.