Various amidinonaphthols and their acyl derivatives were synthesized and evaluated for inhibitory activities against trypsin, plasmin, kallikrein, thrombin, Clr and Cls, as well as against in vitro complement-mediated hemolysis. 6-Amidino-2-naphthyl 4-guanidinobenzoate (74, FUT-175) was found to have potent inhibitory activities, particularly against trypsin (IC₅₀ : 0.02 μM), Clr (IC₅₀ : 0.1 μM) and Cls (IC₅₀ : 0.02 μM), and to be highly effective in inhibiting the complement-mediated hemolysis (IC₅₀ : 0.03μM). FUT-175, furthermore, showed considerable effectiveness in the systemic Forssman shock reaction, in which involvement of the complement system as the pathogenetic factor is well established.