Abstract Neutralization of the lethal, haemolytic and dermonecrotic effects of the venom of the cubomedusan jellyfish Chiropsalmus quadrigatus by antivenenes prepared against the milked venom of Chironex fleckeri was studied. In vitro neutralization of all three properties was obtained, and passive immunization of mice with the antivenene reduced dermonecrosis but did not protect against the lethal effect. Whole tentacle extracts of Chironex fleckeri and of an unidentified tropical cubomedusan, which may contain additional toxins from the tentacle substances, were neutralized by the antivenene. It is concluded that the antivenene would be useful for treatment of envenomation by either of the related cubomedusan species dangerous to man. Venoms of two other jellyfish species, Physalia physalis and Chrysaora quinquecirrha , which have toxic effects similar in some respects to the box jellyfish, were not neutralized by the antivenene.