The cholinesterase activity of the sera of 25 subjects diagnosed as homozygotes for the silent cholinesterase gene was studied by a sensitive enzymatic method employing several thiocholine esters and various inhibitors, and by disc electrophoretic, immunochemical, and chromatographic methods.(a) With one exception, the sera fell into two classes by all criteria. One class (type I, 16 cases) had no normal serum cholinesterase. The other class (type II, eight cases) had about 2% of apparently normal serum cholinesterase. The remaining serum was intermediate between the two classes in several respects. One explanation for these results is that there are several "silent" genes concerned; possibly these are allelic.(b) Normal sera and all silent sera contain small amounts of a cholinesterase activity labeled the residual cholinesterase. The enzyme(s) responsible has properties similar to those of acetylcholinesterase rather than serum cholinesterase. It is estimated that about 1% of the activity of normal serum against acetylthiocholine is due to this enzyme. The source of the residual cholinesterase is not yet known.