Activity of pseudocholinesterase (acylcholine-acyl-hydrolase) elevated up to four times has been detected in sera of members of two German families. The catalytic concentrations of the pseudocholinesterase of the afflicted members of both families (male and female) varied between 4800 U/l and 10200 U/l (acetylthiocholine iodide substrate). The pseudocholinesterase of the propositi exhibits isoenzyme separation patterns in polyacrylamide electrophoresis as well as in electrofocussing which are different from those of pseudocholinesterase from normal persons. No differences could be seen as regards the Km of substrates or the inhibition by dibucaine, fluoride or succinyldiocholine.