Abstract: Enzyme kinetics investigation revealed that the affinity of gut carboxylesterase of Periplaneta americana (L.) for dipterex was 23 times greater than that with the cholines-terase. Addition of purified enzyme preparation of one fraction of the isoenzymes of carboxylesterases with an activity 21-fold strengthened to the test system caused decrease in the binding of cholinesterase with dipterex. This illustrated that the former exerted a sparing effect in favor of cholinesterase. This protective effect ran in parallel with the amount of the enzyme preparation added (in our experiment the increment in the enzyme activity was 0.21, 0.43 and 0.65 times). This survey conformed with the "competitive combination" hypothesis postulated by Motoyama, et al. Since purified enzyme preparation was used in the investigation, this argued much stronger for the affirmation of the hypothesis.