Although CSL box jellyfish antivenom (AV) remains the primary treatment for Chironex fleckeri envenoming, there has been considerable debate regarding its clinical effectiveness. Animal studies have shown that AV is largely ineffective in preventing C. fleckeri-induced cardiovascular collapse. This study examined the effectiveness of CSL box jellyfish AV (ovine IgG), raised against milked venom, and polyclonal rabbit IgG antibodies (Ab) raised against nematocyst-derived venom. A venom dose of 30mug/kg, i.v., which causes an initial presser response (34+/-5mmHg; n=7) followed by cardiovascular collapse, was used in all experiments. A bolus dose of AV (3000U/kg, i.v.) or Ab (12mg; i.e. an equivalent protein load to 3000U/kg AV), administered 15min prior to a bolus dose of venom, did not significantly attenuate the effects of venom. The venom response was also not significantly attenuated when AV (3000U/kg) was given as a bolus dose 10-60min prior to venom infusion. However, when the venom was incubated with either AV (3000U/kg) or Ab (12mg) for 3h prior to infusion, the effect of the venom was almost abolished. The results of this study demonstrate that antibodies raised against both milked and nematocyst-derived venom are able to neutralise the cardiovascular collapse produced by the venom. However, large amounts of AV are required and must be preincubated with the venom to be protective. This indicates a very rapid action of the toxin(s) and that AV is unlikely to be clinically effective because it cannot be administered early enough.