Indirect immunofluorescence to detect the expression of specific markers in primary muscle cells cultured for 35 to 40 days in proliferation medium. Staining of core myofibrillar proteins [myosin type I slow (A) and alpha-sarcomeric actin (B)] produced banded filament images. Staining of myosin type II fast (C) and alpha-sarcomeric actinin (D) produced a continuous filament pattern. The staining pattern produced by spectrin (E) and dystrophin (F) was limited to the periphery of cells and to membrane patches. FITC labeling with Evans blue counterstaining. Original magnification, 60×.