The inhibitory effect of serum is one of the main obstacles to the in vivo use of cationic liposomes as a DNA delivery system. We have found that a novel liposome formulation, DODAC:DOPE (1:1) is totally resistant to the inhibitory effects of serum for transfection of cultured myoblasts and myotubes. Transfection with a lacZ reporter gene in the presence of 95% fetal bovine serum gave up to 25% beta-gal-positive cells in C2C12 myoblasts and about six-fold less in primary human myoblasts. The lower transgene expression in primary cells does not appear to be a result of less DNA uptake but might result from differences in intracellular trafficking of the complexes. DODAC-based liposomes are unique in their resistance to serum inhibition and may therefore be valuable for the systemic delivery of genetic information to muscle and other tissues.