AbstractIncidences of increasing jellyfish mass occurrences necessitate fundamental investigation on their distribution, development and impacts. The potential toxic activity of the giant jellyfish Nemopilema nomurai has been determined by means of in vitro assays using fish gill cells and erythrocytes as responsive elements. Fishing tentacle and oral arm tissues were used for venom analysis. The whole tissue was lyophilized in order to ensure an efficient handling and storage. For testing the toxic activity, the dried material was resuspended and the intact nematocysts were discharged by ultrasound. Four different types of nematocysts have been described using light microscopy by means of their length, width and length to width ratio. The fish gill cells showed a remarkable high susceptibility to N. nomurai venom. A five times increased toxic activity has been measured compared to the boreal Scyphozoa Cyanea capillata corresponding to an EC50 value of 1.5 µg total venom protein per mL cell culture medium. The haemolytic activity of N. nomurai venom was also potentiated compared to C. capillata venom and was determined as HE50 80.2 µg mL-1.