The equilibrium adsorption and binding of the delta-endotoxin proteins (the protoxins and toxins) from Bacillus thuringiensis subsp. kurstaki were 3-5 times as great on montmorillonite as on kaolinite. Maximum adsorption occurred within 30 min (the shortest interval measured), and adsorption was not significantly affected by temperatures between 7 and 50 deg C. The proteins were more easily desorbed from kaolinite than from montmorillonite; they could not be desorbed from montmorillonite with water or 0.2% sodium carbonate, 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; no significant modifications occurred in their structure as the result of binding on the clay, and there was no significant expansion of the clay. The bound proteins appeared to retain their insecticidal activity against 3rd-instar larvae of the noctuid Trichoplusia ni.