We have tried to detect genes which are expressed in normal cells and have the potential of suppressing transforming activity of the Kirsten sarcoma virus oncogene, v-Ki-ras. The experiments involved isolation and characterization of flat revertants from v-Ki-ras-transformed NIH/3T3 cells following either chemical mutagenesis or transfection of a cDNA expression library constructed from total mRNA of normal human fibroblasts. By these procedures multiple genes could be detected which, when transcriptionally activated, suppress different spectra of transformation-associated properties in spite of persistent expression of the v-Ki-ras gene product.