Sera of various phenotypes at serum cholinesterase locus 1, including the newly recognized phenotypes E1 aE1j, E1 uE1j, and E1 fE1J, were studied by immunodiffusion and rocket immunoelectrophoresis. The sera containing the E1j allele show reduced numbers of immunologically active cholinesterase molecules. This finding is consistent with the previously advanced hypothesis that E1j results in reduced numbers of circulating 'usual' (E1u) molecules. Whether this reduction is the result of the low rate of synthesis or of an increased rate of degradation of the cholinesterase remains to be determined.