Blood and saliva from 456 unselected blood donors from the Siriraj Hospital, Bangkok were studied. The frequencies of the ABO and Rh groups closely resemble previous reports but A2 and Cw were seen. The high frequency of M (67.87 per cent) and the low concentration of S (8.33 per cent) make MS and NS fairly uncommon. The frequency of 14.54 per cent for P1 resembles the figure for New York Chinese. Evidence for the presence of Jk was found in three of the 15 families studied but no Jk(a—b—) phenotypes were encountered. The 30.9 per cent frequency of Le(a+b—) and the 40.05 per cent frequency of Se were unexpected, as was the presence of aberrant secretors of ABH. Others findings include 1.19 per cent Mi(a+); 0.94 per cent Mt (a+); 0.44 per cent Lu(a—b—); 1.22 per cent Ve(a-); 2.33 per cent Di(a+) and 13.48 per cent Do(a+).