Abstract In vitro cross-neutralization of venoms of nine species of sea snake was studied, using mice and immunodiffusion patterns as indicators. Tiger snake antivenene was more effective than sea-snake antivenene against all species. Both antivenenes neutralized all venoms tested— Aipysurus laevis , Astrotia stokesii , Enhydrina schistosa , Hydrophis cyanocinctus , H. elegans , H. major , H. spiralis , Lapemis hardwickii , Laticauda semifasciata and Notechis scutatus . Immunodiffusion patterns did not always reflect the in vivo observations. The significance of the results in indicating the choice of an antivenene for treatment of sea snake envenomation is discussed.