Cnidarians contain different toxins causing cytotoxic and cytolytic effects probably due to cell membrane alterations. Mitochondria were used here as an experimental model system to characterize the action of cnidarian toxins. Toxin from the sea anemone Anemonia sulcata affects the function of rat liver mitochondria. Effects are dose- and time-dependent and they disappear when Ca2+ is absent. This toxin could therefore have a phospholipase-like action, as reported for other cnidarians. Mitochondrial alterations by cnidarian toxins could therefore be a particular case of a general mechanism of toxicity directed towards biological membranes.