In previous cytogenetic studies, trisomy for 3q was found to be the most frequent chromosome change associated with induced tumorigenicity by a variety of agents in Chinese hamster cells. Here we describe similar chromosome changes in 11 lines of CHEF/18 cells transfected with the mutant c-Ha-ras containing plasmid pSV2gptEJ. All 11 lines contained the transfected EJ gene and expressed increased levels of p21, the EJ gene product. Ten of the 11 lines were tumorigenic and all but 2 of these were trisomic for all or part of 3q. One line remained diploid and was nontumorigenic despite expressing elevated p21. Two tumorigenic lines from "hit-and-run" transfection with pSV2gpt were shown to express only control levels of p21, but they were trisomic for 3q. These results show that increased p21 expression is neither necessary nor sufficient for inducing tumorigenicity of CHEF cells. We propose that tumorigenicity in the transfected CHEF/18 cells of this study was induced by chromosome rearrangements, especially trisomy for 3q, that occurred at increased frequencies following transfection with pSV2gptEJ.