AbstractInhibition of thermolysin andBacillus brevisneutral proteinase (NP) reactions with N‐(3‐(2′‐furyl)acryloyl)glycyl‐L‐leucinamide by organic solvents and some other simple organic compounds (alkanols, aromatic compounds etc., in total 33 substances) was studied. The enzymes showed very similar affinities for the inhibitors in the whole range of Kivariation (3.7 orders of magnitude). A linear dependence with a nearly unitary slope between the pKivalues and log P's of the compounds was found. The inhibitory power of the compounds, whose molecules are longer than those of propanol or benzene, depended on their ability to donate a hydrogen bond. pKi's of the compounds, in which substituents attached to the propyl group or benzene ring are unable to donate the hydrogen bond, fell off the correlation line; the deviation was proportional to the van der Waals volume of the substituent. pKi's of tert‐butanol and cyclohexanol also fell off the correlation line. Geometrical thickness of their molecules exceeds that of the other studied inhibitors; this may explain their lowered affinity for the enzymes. 6,6‐ or 7,5‐bicyclic compounds did not inhi