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Myotonic dystrophy (DM) is the most common form of inherited neuromuscular disease in adults and is characterized by progressive muscle wasting and myotonia. The mutation responsible for DM has been identified as the amplification of apolymorphic (CTG)$\rm\sb{n}$ repeat in the 3$\sp\prime$ untranslated region of a gene encoding a serine/threonine kinase (DMK). We have produced a polyclonal rabbit antibody preparation against a fusion protein encoding C-terminal amino acids 471-629 of the human DMK gene. This antibody specifically detects products of both full length and truncated human DMK genes expressed in bacteria and in insect cells. On immunoblots, we observed protein species of $\sim$74 and 82 kDa in human and rodent cardiac muscle and skeletal muscle, as well as rodent ependyma and choroid plexus. By immunofluorescence, DMK was found to localize postsynaptically at the neuromuscular junction of skeletal muscle, at intercalated discs of cardiac tissue and at the apical membrane of the ependyma and within the choroid plexus. We have also detected 2-3 species ($\sim$45-50 kDa) in brain tissue. Neuroanatomical evidence suggests synaptic localization for DMK in rodent cerebellum, hippocampus, midbrain and medulla. These results indicate that DMK may have a role in intercellular communication. Finally, we have demonstrated that DMK is present in adult and congenital DM tissues and that its distribution is no different than that observed in normal controls.