DNA prepared from the 3-methylcholanthrene (3MC)-transformed human 312H cell line induced foci on NIH/3T3 cells, whereas DNAs prepared from 7,12-dimethylbenz[a]-anthracene-transformed and the dimethylsulfoxide control 312H cell lines failed to induce foci. The transformed gene from the 3MC-transformed 312H cells was identified as an activated form of the human cellular transforming H-ras oncogene. Analysis of the ras oncogene p21 product in this transformant by immunoprecipitation and gel electrophoresis suggested that this gene was activated by mutation in the 61st codon. These findings demonstrate that activation of a member of the ras gene family can occur in a chemically transformed human cell line.