SMTP-7 is a small molecule that promotes the proteolytic activation of plasminogen by relaxing its conformation. SMTP-7 has excellent therapeutic activities against thrombotic stroke in several rodent models. The objective of this study was to elucidate detailed mechanism of the action of SMTP-7 in vitro. We report here that the action of SMTP-7 requires a cofactor with a long-chain alkyl or alkenyl group, and that the fifth kringle domain (kringle 5) of plasminogen is involved in the SMTP-7 action. In this study, we found that the SMTP-7 action to enhance plasminogen activation depended on the presence of a certain type of surfactant, and we screened biologically relevant molecules for their cofactor activity for the SMTP action. As a result, phospholipids, sphingolipids, and oleic acid were found to be active in assisting the SMTP-7 action. On the contrary, stearic acid and bile acids were inactive. Thus, a certain structural element, not only the surface-activating potential, is required for a compound to act as a cofactor for the SMTP-7 action. The plasminogen molecule consists of a PAN domain, five kringle domains, and a serine protease domain. The cofactor-dependent effects of SMTP-7 was observed with plasminogen species including kringle 5 such as intact plasminogen (Glu-plasminogen), des-PAN plasminogen (Lys-plasminogen), and des-[PAN − (kringles 1–4)] plasminogen (mini-plasminogen). However, SMTP-7 effect was not observed with the smallest plasminogen species des-[PAN − (kringles 1–4) and a half of kringle 5)] plasminogen (micro-plasminogen). Thus, kringle 5 is crucial for the action of SMTP-7.