Northern blot hybridization experiments showed that Apolipoprotein A-I (Apo A-I) mRNA is present at high concentration in chicken myotubes cultured in vitro, while it is virtually absent in fibroblasts and myoblasts. Myotubes are also capable of translating and secreting in the culture medium a protein which is specifically immunoprecipitated by anti-Apo A-I antibodies and has the same electrophoretic mobility as Apo A-I purified from circulating high-density lipoproteins. The appearance of Apo A-I mRNA in myotubes depends on the transcriptional activation of the corresponding gene, as it was shown by hybridizing 32P-labeled RNA synthesized in isolated nuclei to Apo A-I cDNA. The activation of the Apo A-I gene is regulated by the muscle cell coordinately with muscle-specific genes. In fact, treatment with TPA, a powerful inhibitor of differentiation, efficiently prevents myoblasts from producing Apo A-I mRNA, as well as muscle actin mRNA, and causes myotubes to quickly cease Apo A-I mRNA synthesis. The existence of a strict relationship between Apo A-I mRNA concentration and myogenic cell differentiation was also confirmed by experiments with quail myoblasts transformed with a temperature-sensitive mutant of the Rous Sarcoma Virus. Cells raised at the permissive temperature (undifferentiated phenotype) do not contain Apo A-I as well as alpha-actin mRNAs, while shifting to the nonpermissive temperature (differentiated phenotype) causes a rapid increase in Apo A-I and alpha-actin mRNA concentration.