Abstract The expression of three c- onc genes (c- erb, c- myc, c- myb) was investigated in five cell lines established from fibrosarcomas induced with 20-methylcholanthrene (MCA) of Japanese quails. These cell lines showed low levels of the three c- onc genes, with the exception of two cell lines that accumulated moderate (MCAQ 1–4) and large amounts (MCAQ3-5) of c- myc RNA. Molecular cloning and restriction endonuclease analyses indicated that expression of c- myc in these two cell lines were not associated with detectable rearrangements in the c- myc locus, that the size of the c- myc transcript (2.7 kb) in MCAQ3-5 was similar to that of the normal c- myc messenger RNAs (mRNA) and that the transcriptional activation observed in MCAQ 3–5 was not mediated by the LTR (long terminal repeat) of a proximate ALV (avian leukosis virus) provirus. Finally, when analysed with the restriction enzymes MspI and HpaII, the c- myc locus of MCAQ3–5 and MCAQ 1–4 was found hypomethylated as compared with that of the other cell lines tested that show low levels of c- myc transcripts. Our results suggest that one of the ways methylcholantrene could mediate transformation is by inducing an abnormal regulation of the c- myc gene.