The equilibrium adsorption and binding of the delta-endotoxin proteins, i.e., the protoxins (M(r) = 132 kDa) and toxins (M(r) = 66 kDa), from Bacillus thuringiensis subsp. kurstaki were greater on montmorillonite than on kaolinite (five-fold more protoxin and three-fold more toxin were adsorbed on montmorillonite). Approximately two- to three-fold more toxin than protoxin was adsorbed on these clay minerals. Maximum adsorption occurred within 30 min (the shortest interval measured), and adsorption was not significantly affected by temperatures between 7-degrees and 50-degrees-C. The proteins were more easily desorbed from kaolinite than from montmorillonite; they could not be desorbed from montmorillonite with water or 0.2% Na2CO3, but they could be removed with Tris-SDS (sodium dodecyl sulfate) buffer. Adsorption was higher at low pH and decreased as the pH increased. Adsorption on kaolinite was also dependent on the ionic nature of the buffers. The molecular mass of the proteins was unaltered after adsorption on montmorillonite, as shown by SDS-PAGE (polyacrylamide gel electrophoresis) of the desorbed proteins; no significant modifications occurred in their structure as the result of binding on the clay, as indicated by infrared analysis; and there was no significant expansion of the clay by the proteins, as shown by x-ray diffraction analysis. The bound proteins appeared to retain their insecticidal activity against the third instar larvae of Trichoplusia ni.