Abstract As a first approach to study the cellular events involved in myotonic dystrophy, we have produced a polyclonal antibody against a peptide sequence of the predicted gene product. This antibody specifically recognizes a 54 kDa protein in human skeletal muscle. This protein phosphorylates a co-polymer Glu/Tyr but not Myelin Basic Protein. This indicates that the myotonin-protein kinase has a tyrosine kinase activity in human skeletal muscle. This is the first demonstration of the kinase activity of the myotonin-protein kinase.