When smooth muscle myosin light chain kinase, purified by standard procedures from chicken gizzard smooth muscle, was applied to an anion-exchange high-performance liquid chromatographic column, three well resolved peaks were obtained. Each peak contained a single protein whose electrophoretic mobility corresponded to that of MLCK. However each enzyme was characterized by a different specific activity. Peptide mapping experiments were unable to demonstrate different proteolytic patterns for the three proteins. Treatment of myosin light chain kinase with alkaline phosphatase, prior to ion chromatography, resulted in a change of elution profile. These experiments suggest that myosin light chain kinase could exist in three forms characterized by a different degree of phosphorylation.