HI-6 and TMB-4 were the most effective and safe of 7 cholinesterase reactivators tested as agents for the prophylaxis of proserine poisoning of male mice. The reactivator HI-6 strongly potentiated the prophylactic efficacy of a mixture of atropine and arpenal administered in the doses sufficient for the blockade of both the m- and h-cholinoreactive systems of mice. As demonstrated by experiments in vitro, HI-6 and TMB-4 did not reacivate proserine-inhibited cholinesterase. The natural anticholinesterase activity of HI-6 was negligible. Based on the correlation of the data obtained to the reported data indicating that HI-6 has a low ganglioblocking activity it is inferred that the direct effect on the receptor is of no importance for the potentiating effect. It is assumed that HI-6 modulates the cholinoreactive systems, which leads to a dramatic increase of the efficacy of cholinolytics.