S-2765, S-2366, and S-2288, S-2403 are suitable for single-chain tPA, and S-2251 is used in Coaset tPA, but there are no substrates specifically for two-chain tPA. A paper by Verheijen et al. (Thromb Res, 1985; 39: 281 – 288), however, describes a method comparing the direct amidolytic activity of tPA on S-2366 and the plasminogen activating activity. Also, the substrate S-2288 is suitable for measuring double-chain tPA because it has a slightly higher sensitivity than S-2366. S-2288 should be used with purified systems, though, since this substrate is sensitive for several proteases.