The electrophysiological effects of both trypsin-activated native and 42- and 51-kDa cloned binary toxins of Bacillus sphaericus were investigated on cultured Culex quinquefasciatus cells using the patch-clamp technique. Rates of reduction in whole-cell membrane resistance were correlated with increasing native toxin concentration. The 42- or 51-kDa cloned toxin alone at 50 IJ;mu g/ml reduced the resistance. Electrophysiological effects occurred before any changes were visible by phase-contrast microscopy. (C) 1997 Academie Press.