RNase inhibitors are commonly used to block the RNase activity in manipulations with RNA-containing preparations. Recently RNase inhibitors, either synthetic or natural, have been intensively sought because they appeared to be promising for therapy of cancer and allergy. However, there is only a limited number of efficient RNase inhibitors. We have shown that a low molecular weight chitosan (M-r similar to 6 kDa) inhibits activity of pancreatic RNase A and some bacterial RNases with inhibition constants in the range of 30-220 nM at pH 7.0 and ionic strength 0.14 M. The preferential contribution to the chitosan complex formation with RNases is due to establishment of 5-6 ion pairs. The results of this work show that polycations may efficiently inhibit ribonuclease activities. (c) 2007 Elsevier Inc. All rights reserved.