C2 toxin (C2T) elaborated by Clostridium botulinum types C and D is composed of two nonlinked protein components, designated components I and II. The toxin, a mixture of untrypsinized component I and trypsinized component II, induced marked morphological changes of tissue-cultured cynomolgus monkey kidney cells; the characteristic response of the cells to the toxin was rounding, which increased proportionally to log dose of the toxin. The components alone and a combination of untrypsinized components I and II showed little activity. The rounding of the cultured cells was not accompanied by inhibition of protein and nucleic acid syntheses of the cells, although the rounded cells ultimately lost viability. Immunofluorescence studies showed that component II, either trypsinized or untrypsinized, bound to the cell surface, whereas component I bound to the cells only in the presence of trypsinized component II. The present results support the previously proposed idea concerning the mode of action of C2T, that components I and II of C2T act together as a molecule with dual functions; component II as the recognizer of the receptor site on the cell surface membranes and component I as the effector in the cytoplasm by preferential inactivation of cytoskeletal actin, which results in alteration of cell morphology, and subsequently in cellular damage.