We have identified a rat smooth muscle cell mRNA, 2B7, which distinguishes smooth muscle cell phenotypes in vitro. Sequence and tissue distribution data strongly suggest this mRNA to be identical to osteopontin (secreted phosphoprotein I, 2aR). In vivo, 2B7 mRNA is expressed in normal rat aorta and carotid arteries at levels 50-60-fold greater than heart, and about 3-4 times the levels found in adult rat kidney on a per micrograms of total RNA basis. Of the other smooth muscle sources surveyed, significant levels of 2B7 mRNA were detectable in total RNA prepared from rat uterus and stomach, but not intestine. 2B7 mRNA levels in both carotid and aortic artery increase with age, and are elevated approximately 5-fold in the carotid artery 48 h after balloon angioplasty. The presence of osteopontin (secreted phosphoprotein I, 2aR) in the normal artery wall and its increased expression after injury suggests a previously unappreciated role for this molecule in the vascular system.