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.