The bis-pyridinium oxime HI-6, in conjunction with atropine, was found to offer significant protection against multiple LD50 challenges with the organophosphorus compounds soman and tabun. In adult rhesus macaques, the therapeutic administration of HI-6 with atropine and diazepam protected three of four animals from the lethal effects of 5 x LD50 of soman and three of three animals from 5 x LD50 of tabun. However, when toxogonin was substituted for HI-6 in the therapeutic mixture, all three animals poisoned with 5 x LD50 of soman died. In rats, the 24 h protective ratios against tabun and soman with HI-6 were 2 and 3.5, respectively, whereas in guinea pigs these values were between 4 and 6 for both agents. No evidence was obtained for acetylcholinesterase (AChE) reactivation by HI-6 in tissue from tabun-poisoned rodents or following soman or tabun in primate plasma. The results underscore the significant therapeutic benefit of HI-6 in primates, a species specific efficacy against tabun, and argue for some mechanism of action of HI-6 at least partly unrelated to AChE reactivation.