Dibucaine number (DN) and fluoride number (FN) of the recombinant 330 I mutant ChE (r330 I) expressed in human kidney cells (293 cell) were compared with recombinant usual ChE (rUU), by several assay kits and substrates. All of them showed lower the values compared with rUU. However, the r330 I/rUU ratios about FN determined by several substrates were higher than that determined by propionyl thiocholoneiodide (PTCI), which was recommended by American Association for Clinical Chemistry. In conclusion, commercially available assay kits may not be suitable for the determination of L330 I.