A simple, time-saving agar gel electrophoretic technique for determination of adenylate kinase phenotypes is described. The separations were performed in 0.05 m phosphate buffer, pH 6.2, in a water-cooled apparatus at a voltage gradient of 22 V/cm. Thirty samples could be separated in 30 min. The resolution was at least as good as with starch gel electrophoresis. Investigation of hemolysates from 505 blood doners in southern Sweden revealed a frequency of 0.9653 for the AK1 gene and 0.0347 for AK2. None of the rare phenotypes were found.