The pharmacological and biochemical isolation of cnidarian venoms has been hindered by difficulties with both extracting pure venom from nematocysts and venom stability. The development of a new technique to extract active, pure venom of Chironex fleckeri and Chiropsalmus sp. has enabled identify both neurotoxic and myotoxic activity in their venoms. These activities are similar, but not identical in each species. Venom (50 micro g/ml) from both species significantly inhibited indirect and direct twitches of the chick biventer nerve-muscle preparation. Pre-incubation with 1U/ml box jellyfish antivenom did not have any significant effect on venom-induced reductions of indirect twitches. However, this activity was markedly attenuated by prior addition of 5U/ml antivenom, albeit to a lesser degree for Chiropsalmus sp. In contrast, prior addition of 5U/ml box jellyfish antivenom did not neutralise the myotoxic activity of C. fleckeri venom (50 micro g/ml), although it did inhibit the myotoxicity produced by Chiropsalmus sp. venom (50 micro g/ml). Antivenom (5U/ml) added 1h after the addition of C. fleckeri venom (50 micro g/ml) had no effect on the indirect or direct twitches of the skeletal muscle preparation. However, it partially restored the reduction in indirect twitch height caused by Chiropsalmus sp. venom (50 micro g/ml). Myotoxicity was confirmed in muscle preparations stained with hematoxylin and eosin.Therefore, although antivenom was able to neutralize the neurotoxic effects of both species, and the myotoxic effects of Chiropsalmus sp., when added prior to venom, it was unable to reverse the effects after venom addition. This suggests that antivenom is unlikely to be useful in the treatment of neurotoxic or myotoxic effects in patients, although these effects are rarely seen clinically.