Abstract: This article presents the results of 39 determinations of myoglobin (Mb) with radioim-munoassays in sera from 28 patients with dermatomyositis. Positive results for Mb(> 50ng/ml) were found in 16 sera (41.0%). In the active cases the percentage of positive results was even higher (48.5%). We found the Mb radioimmunoassay to be more sensitive than the SGPT and LIiH determination, and its sensitivity not lower than the 24 hour urine creatine and SGOT determination, run concurrently on these sera. This suggests that Mb radioimmunoassay may be valuable as an adjunct,in the diagnosis of dermatomyositis.