Elemental sulfur is a specific and potent inhibitor of the muscle-type isoenzyme of adenylate kinase (EC 2.7.4.3). We find inhibition by sulfur and by diadenosine pentaphosphate to be similarly potent and specific. Some properties of inhibition of adenylate kinase isoenzymes by sulfur are given. The adenylate kinase isoenzymes from skeletal muscle, brain, and heart muscle are inhibited by sulfur; those from liver and kidney are not. Other enzymes not inhibited by sulfur include the isoenzymes of creatine kinase (EC 2.7.3.2). We show that creatine kinase can be measured in serum when adenylate kinase is inhibited by sulfur, and that the sensitivity and specificity of this inhibition are of the same order as the inhibition of serum adenylate kinase activity by AMP plus diadenosine pentaphosphate.