Published sequence data of the human c-myc gene indicate the presence of a coding capacity for a polypeptide of 188 residues within the first exon. Using antibodies raised against five synthetic peptides corresponding to different non-over-lapping parts of this polypeptide, two proteins of 32 kd and 58 kd antigenically related to the synthetic peptides have been detected in extracts of human cells. The confidence of this detection has been reinforced by showing that epitopes corresponding to different peptides were indeed located on the same molecule and that the 58 kd protein appears to be a dimeric form of the 32 kd protein. That these proteins originate from the first exon was indicated by: hybrid-arrested translation experiments followed by immunodetection of the translation products; in vitro translation of messenger RNA derived from cloned exon 1 by SP6 polymerase transcription.