Overlapping cDNA clones encoding bovine osteonectin were isolated from a lambda gt11 expression library constructed from bovine bone cell mRNA. The longest clone, lambda On 17 (insert size 2.0 kb) was studied in detail. The clone was shown to encode osteonectin by hybrid select translation experiments and by DNA sequence analysis. Northern analysis of bone cell RNA showed the length of the osteonectin mRNA to be 2.0 kb. Osteonectin message was found in bone but not in soft tissue (liver and brain) preparations consistent with the distribution of the protein in these tissues. On the other hand, osteonectin message was observed in tendon, a tissue in which little or no osteonectin protein is found in vivo. Hybridization of osteonectin cDNA was detected in cells from a number of species including human, rat, mouse and chick. The level of osteonectin mRNA was drastically decreased in chick embryo fibroblasts transformed by Rous sarcoma virus.