The lck proto-oncogene, a member of the src gene family, encodes a lymphocyte-specific protein tyrosine kinase (p56^lck) that is implicated in the pathogenesis of lymphoid neoplasia^1–4. We report here that 5' lck sequence elements, containing AUG codons, significantly reduce the in vivo efficiency of p56^lck translation from the normal messenger RNA. This result provides a quantitative explanation for the overexpression of p56^lck in two retrovirally-induced murine lymphomas that express abnormal lck transcripts from which the physiological 5' translational control region has been deleted and non AUG-containing viral sequences substituted. In contrast to other mammalian genes, most proto-oncogenes contain AUG codons 5' to the authentic initiation codon⁵, suggesting that cells can regulate translational start sites in these mRNAs. Abrogation of translational control may be a general mechanism of proto-oncogene activation in malignancy.