A monoclonal antibody (12D11) is presented that binds an epitope on histone H1 only when it is phosphorylated. Skeletal myoblasts, which are cultured in high mitogen medium to induce proliferation and inhibit differentiation, contain histone H1 reactive with monoclonal antibody 12D11, whereas differentiated myocytes and adult skeletal muscle do not. Phosphorylation of H1 at the 12D11 epitope, as assessed by antibody binding, is also induced and maintained in myoblasts cultured in low mitogen medium supplemented with transforming growth factor beta, which blocks differentiation but allows the cells to withdraw from the cell cycle (Olson, E., Sternberg, E., Hu, J., Spizz, G., and Wilcox, C. (1986) J. Cell Biol. 103, 1799-1805; Massague, J., Cheipetz, S., Endo, T., and Nadal-Ginard, B. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 8206-8210). These observations suggest that phosphorylation of histone H1 at the 12D11 epitope is associated with the negative regulation of myoblast differentiation by growth factors.