Five differential inhibitors of plasma cholinesterase have been compared using benzoylcholine as substrate. None of the inhibitors (dibucaine, NaF, NaBr, NaCl, or pancuronium dibutyryloxy bromide) could be used singly to resolve all the variants. Better resolution was obtained when two inhibitors were used in conjunction. Clear differentiation of all six genotypes was obtained only with the combined use of pancuronium dibutyryloxy bromide and sodium fluoride. The limitations of some of the parameters are discussed.