Testican is a highly conserved, differentially expressed gene product of unknown function. Since testican is expressed by human endothelial cells and includes a signal sequence, it was our hypothesis that testican protein would be present in blood. We have developed chicken antibodies specific for testican sequence near the N-terminal and identified a 130-kDa form of testican in human plasma. This is much larger than the calculated molecular weight of the encoded polypeptide, suggesting glycosylation of this plasma protein, and large forms of recombinant testican produced in culture were found to include chondroitin sulfate. The 130-kDa form of testican is unstable in plasma. It is converted to smaller stable forms by separable plasma factors that can be blocked by certain serine protease inhibitors, Testican size conversion may be important in its functional activation or decay. One testican domain has strong homology to thyropin-type cysteine protease-inhibitors. Thus, testican may have a function related to protease inhibition in the blood.