The protein content and the rates of hydrolysis of p-nitrophenyl palmitate (pNPP) in water (soluble enzyme and emulsified substrate) and in heptane (soluble substrate and insoluble enzyme) were measured for thirty-two commercial lipase preparations. The protein content of the powders varied in a wide range as well as the activity on emulsified pNPP showing the high heterogeneity of the commercial samples. Activity in heptane also varied but to a lesser extent than that in water. There was no direct correlation between activities in water and in heptane as assayed with the same hydrolytic reaction. The ratio of activity in heptane to that in water, R-O/A ratio, was introduced to characterize activity in organic media. Six lipases showed R-O/A values higher than 1 demonstrating a higher activity in organic solvent than in water. A linear correlation of R-O/A with activity in water (log plot) suggested the strong influence of diffusional limitations on activity of solid enzyme suspended in organic solvents. (C) 2001 Elsevier Science Inc. All rights reserved.