The replacement of hydrogen atoms by deuterium in hydrogen bonds of base pairs AT and GC decreases the rate of unwinding DNA by more than 30% per each unzipped base pair. In active helicases this isotope effect refers to the ratio of the rate constants for unzipping closed base pairs in protiated and deuterated DNA. In passive helicases the effect is controlled by ratio of equilibrium constants for opening and closing base pairs in protiated and deuterated DNA. Hydrogen/deuterium isotope effects on the unwindening of double strand DNA seems to explain, at least partly, biological and pharmacological effects of heavy water on living organisms and may be used as a means to explore new facets of the helicase functioning.