The function of the c-myc protein, the product of a proto oncogene, is not clearly understood although many reports, including ours, suggest that the c-myc protein plays several roles in the regulation of transcription and DNA replication. Here we examined the effects of c-myc protein on transcription from the c-myc promoter, and by inference its role in auto-regulation, after introducing into cultured cells a c-myc expression vector and a CAT reporter gene linked to the promoter and upstream region of the human c-myc gene. To minimize the effects of the endogenous c-myc protein on the exogenously added CAT reporter gene, the transfected cells were treated under serum-free conditions. The results show that CAT expression from the myc promoter increased in a dose-dependent manner after addition of the c-myc expression vector, and that it also required the presence of a c-myc binding sequence previously identified 2 kb upstream from c-myc's first exon. Moreover, the domains of the c-myc protein important for transactivation were determined by use of various deletions mutants of c-myc cDNA. The results showed that the N-terminal portion in the c-myc protein was necessary for transactivation beside the C-terminal portion containing basic region, helix-loop-helix, and leucine zipper.