The mechanism by which pertussis toxin induces morphological changes in Chinese hamster ovary cells was studied to determine whether the resulting clustered growth pattern is due to toxin-catalyzed ADP-ribosylation of a cellular substrate. While pertussis toxin was extremely potent in inducing morphological changes in Chinese hamster ovary cells, preparations of isolated A subunit or B oligomer exhibited greatly reduced activity. The clustered growth response of these cells correlated with ADP-ribosylation of a 41-kilodalton cellular substrate for the toxin in that the toxin concentration and time of exposure to the toxin required for ADP-ribosylation were the same as those needed for alterations in cellular morphology. Moreover, pertussis toxin modified by either chemical or photolytic methods exhibited similar decreases in the ability to ADP-ribosylate the cellular substrate and alter cell morphology. These results suggest that clustering of Chinese hamster ovary cells is due to toxin-catalyzed ADP-ribosylation of a 41-kilodalton substrate. Therefore, alteration in Chinese hamster ovary cell morphology can be used as a measure of toxin activity. This assay should prove to be a useful tool in the development and evaluation of new pertussis vaccines.