The isozymes of serum cholinesterase are shown to be interconvertible. Concentration of the individual isozymes results in an electrophoretically slower functional enzyme unit, whereas dilute solutions result in functional units with greater mobility. Since each of the four most mobile isozymes when isolated is convertible to the major isozyme form and each slower isozyme when isolated can be converted into the more mobile forms, this sequence is most likely stepwise in nature. The results indicate that the isozymes of serum cholinesterase are a manifestation of a polymerization sequence.

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Intra-individual variation of serum cholinesterase activity as determined in 82 normal adults studied 373 times over periods up to 5 years is found to be relatively low. The over-all coefficient of variation was 8.4, the linear correlation between the first and subsequent tests in the same person was r = .87 (first versus second test in 82 subjects) and r = .90 (first versus seventh test in 14 people). A decline of 0.66 U. from a single prior measurement when in normal health is significant at the .05 level. The clinical significance of the test is discussed.

The evidence is described that serum cholinesterases are a group of discrete isoenzymes, differing primarily in molecular size but also kinetically. A number of genetically determined variants have been described which probably differ by virtue of structural modifications. The clinical importance of the measurement of serum cholinesterase activity is discussed.

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