To identify genes active in cells of the osteoblast lineage we have begun to characterise the phenotype of the rat osteoblast-like osteosarcoma cell line ROS 17/2.8. We have used the method of differential hybridization to identify cDNA clones encoding mRNA species which are expressed in ROS 17/2.8 cells but not in a non-osteoblast-like osteosarcoma cell line (ROS 25/1). We have identified a number of gene products exhibiting this pattern of expression.