The differentiation grade of cells in culture is dependent on the composition of the culture medium. Two commonly used myogenic cell lines, mouse C2C12 and rat L6, usually differentiate at a low concentration of horse serum. In this study we compared the effect of horse serum with a medium containing a low percentage of Ultroser G and rat brain extract. The maturation grade was evaluated on the basis of various biochemical, (immuno)histochemical and cell-physiological parameters. Substitution of horse serum by Ultroser G and rat brain extract during the differentiation phase resulted in a higher maturation grade of the myotubes of both cell lines, on the basis of creatine kinase activity and the diameter of the myotubes. In addition, the C2C12 myotubes display cross-striation, contain a higher percentage of creatine kinase muscle-specific isoenzyme MM, show a ninefold increase in acetylcholine receptor (AChR) clusters, form a continuous basement membrane, and have a lower resting cytosolic Ca2+ concentration. L6 myotubes show a fivefold increase in AChR clusters and a twofold increase in the expression of the mRNA of the epsilon-subunit of AChR.C2C12 cells show spontaneous contraction and response of cytosolic Ca2+ to various stimulants in contrast to L6 cells which do not. These studies established that the Ultroser G/brain extract medium leads to a higher differentiation grade of both cell lines, but parameters appropriate for use as differentiation markers appear to differ between both cell lines.