The effects of montmorillonite (M) or kaolinite (K) on the vertical movement of the insecticidal Cry1Ab protein of Bacillus thuringiensis subsp. kurstaki (Bt) were studied in repacked soil columns. The protein was added to the columns either in a purified form, as root exudates from growing plants of Bt corn, or within the biomass of residues of Bt corn. The soil was amended to 0, 3, 6, 9, or 12% (vol vol-1) with the clays. Vertical movement of the protein generally decreased as the content of either clay was increased, and the amount of protein recovered in leachates increased as the concentration of purified protein added was increased. The largest amount of purified protein (ca. 75%) was leached from soil not amended with clay, whereas the lowest amount (ca. 16%) was recovered from columns containing soil amended to 12% with M or K. The Cry1Ab protein was also present in leachates from soil columns in which various hybrids of Bt corn were grown or to which biomass of Bt corn had been added, whereas it was absent in leachates from columns in which the respective isolines of non-Bt corn were grown or to which biomass of non-Bt corn had been added. The Cry1Ab protein exhibited stronger binding and higher persistence, as well as remaining nearer the soil surface, in soil that contained the higher clay concentrations (i.e. had a higher cation-exchange capacity and specific surface area), indicating that it could be transported to surface waters via runoff and erosion. In contrast, the protein was more readily leached through soil with lower clay concentrations, indicating that it could contaminate groundwater.