Impurities cannot readily be removed during crystallization of avermectin B1a. This study aims to investigate the main impurity effects on the crystallization of avermectin B1a. Impurities, A1a, A2a, B2a and saccharides, were isolated from avermectins crude powder by adsorption column chromatography. Impurity effects on nucleation rate, crystal morphology, and purity of crystals were studied by varying the amount of impurity (0-26.1%, w/w) as well as the relative supersaturation ratio (S, 1.8-2.8). Results showed that A1a, A2a and B2a suppressed primary nucleation. Saccharides promoted nucleation at low supersaturation, while they slightly inhibited nucleation at high supersaturation. A2a and B2a had a marked effect on the crystal morphology, while Ala and the saccharides did not show any significant effect on the crystal morphology. A1a, A2a and B2a could be incorporated into the growing crystals due to the similarity of their structure with the host molecule. (c) 2006 Elsevier B.V. All rights reserved.