Alkaline phosphatase positive mononuclear cells express myogenic regulatory factors. Mononuclear cells were isolated from C2C12 cell cultures after 6 days of differentiation in presence of 30 mM sodium chlorate. Permeabilized cells were fixed and stained for alkaline phosphatase activity using ELF-97 detection kit and for MRFs using polyclonal anti-mouse MyoD, anti-human Myf5 or anti-rat myogenin antibodies. TRITC-conjugated anti-rabbit IgG was used as secondary antibody. Symbols represent: arrow, MRF+/ALP+; arrowhead, MRF+/ALP-; diamond, MRF-/ALP+; asterisk, MRF-/ALP-. Bar = 50 μm. B. Quantification of single or double labeled cells was performed in 10 independent fields. a, MRF+/ALP+; b, MRF+/ALP-; c, MRF-/ALP+; MRF-/ALP-. (Mean ± S.D. of two independent experiments).