Precipitation, one of the steps used most routinely in protein purifications. suffers from a general lack of selectivity. In an attempt to impart controllable selectivity of a definite nature, carboxymethyl cellulose was developed as a soluble ion-exchange precipitant for isolation and purification of proteins. Carboxymethyl cellulose was used as a reversibly soluble/insoluble ion exchange matrix, which after binding proteins from ＇crude＇ could be precipitated quantitatively with a combination of calcium and polyethylene glycol. The precipitated proteins were recovered by solubilising in a dissociating buffer devoid of any one or both of the precipitants i.e. calcium and polyethylene glycol; followed by reprecipitating of carboxymethyl cellulose. Lactoperoxidase was used as a model enzyme and purified from milk whey using the proposed system. A purification factor of about four was achieved in a single step of precipitation with a suitable choice of binding and elution conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.