Osmotic swelling of M. sexta midgut brush border membrane vesicles induced by various concentrations of Cry1Aa. Vesicles equilibrated in 10 mM CAPS-KOH (pH 10.5) and 1-mg/ml bovine serum albumin were rapidly mixed with an equal volume of 150 mM KCl, 10 mM CAPS-KOH (pH 10.5), 1-mg/ml bovine serum albumin, and the indicated concentrations of toxin (in picomoles of toxin per milligram of membrane protein). Percent volume recovery was calculated for each experimental point, and the values measured for control vesicles, assayed in the absence of toxin, were subtracted from those obtained in the presence of toxin. Data are means ± SEM of four experiments. For clarity, error bars are shown for every 100th experimental point.