Guided by genetic studies, human sera were selected and postulated to contain either the usual type of serum cholinesterase (pseudocholinesterase), or an atypical variety of this enzyme, or a mixture of these two enzymes. The shape of concentration-inhibition curves obtained with sera of the last-named category permitted no other interpretation than the presence of a mixture of two enzymes. The concept of the existence of a usual and of an atypical form of human serum cholinesterase was thereby supported. The values for I 50 (concentration of inhibitor causing 50 per cent inhibition) of 16 inhibitors for normal and atypical esterase were compared under standard conditions. Both types of esterase were equally affected by the phosphate inhibitors DFP and TEPP. All other inhibitors blocked the atypical esterase less than the normal one. In relation to their usual blocking capacities, the bisquaternary compounds C 10 and succinylcholine were particularly weak inhibitors of the atypical esterase. For the 12 remaining inhibitors, which included tetramethylammonium, procaine, and eserine, the normal and atypical I 50 could be related by the exponential equation I 50-atypical = (I 50-usual) 0.785; that is, the difference between corresponding I 50-values increased with increasing potency of the inhibitors. Several compounds, e.g., decamethonium and chlorpromazine, were competitive inhibitors of atypical esterase, but they were not completely competitive inhibitors of the usual enzyme. Every fourth molecule of C 10 and of succinylcholine seemed to block two active centres of the normal esterase, but each of these molecules blocked only one active centre of the atypical enzyme.