1.5-OH-Dihydroxyphenylalanin (5-OH-Dopa) was found with the aid of high-pressure liquid chromatography (HPLC) and GC MS to be present in four species of sea anemones.5-S-Cysteinyldopa was also found in the sea anemones as well as in the epidermis of two crustacean species. The results were obtained with HPLC and, in the case of the crustaceans, also verified spectrofluorometrieally.Purified tyrosinase from the sea anemone Metridium senile and the crayfish Pae!fastacus leniusculus had a significant capacity for oxidizing Dopa to 5-S-cysteinyldopa and 2-S-cysteinyldopa in the presence of cystein. In the absence of cystein 5-OH-Dopa was formed only by the sea anemone.