Esterases of 14 strains of K. pneumoniae, 14 strains of K. oxytoca, 16 strains of E. aerogenes and 16 strains of E. gergoviae were analyzed by horizontal electrophoresis in polyacrylamide-agarose gel. Four principal esterase bands (designated E1 to E4) and 9 minor bands differing in their activity towards synthetic substrates and in their sensitivity to heat and to di-isofluoropropyl phosphate were defined. The comparative distribution of bands showed that the 4 spp. analyzed were characterized by distinct electrophoretic patterns of their esterases. Band E1 was found in all 4 spp., bands E2 and E3 only in K. oxytoca and band E4 only in some strains of E. gergoviae. The apparent MW of esterases E2 and E3, determined by electrophoresis in a 4-30% polyacrylamide gradient gel, were 58,000 (.+-. 1000) and 72,000 (.+-. 1800), respectively.