Exposing Chinese hamster ovary cells in culture to pertussis toxin resulted in a novel clustered growth pattern. The specificity of the response for pertussis toxin was shown by neutralization of the activity with specific anti-toxin antibody, heat lability (80 degrees C for 15 min), and absence of such activity by culture media from nontoxigenic Bordetella species. Although a lag of at least 16 h was required before clustered growth was seen, exposure to the toxin for as little as 10 min resulted in a full response 24 h later. The morphological effect appeared to be independent of the cyclic AMP-mediated cell elongation elicited by the heat-labile enterotoxin from Vibrio cholerae in that the pertussis toxin effect was seen in both the presence and absence of elongation. Although the mechanism by which this effect is mediated remains to be determined, it is already providing a useful in vitro assay for pertussis toxin.