Monitoring of eastern blue mussels (Mytilus edulis), contaminated with domoic acid, involved mouse bioassays and quantitative analysis using HPLC. Mice undergo a typical scratching syndrome at sublethal as well as lethal doses of domoic acid. The onset of scratching behaviour and time of death in mice were inversely related to the dosage of domoic acid. An LD50 (i.p.) of 3.6 mg domoic acid/kg mouse was calculated. Toxic mussels held in tanks and flushed with uncontaminated sea water showed a decline in domoic acid concentration in mussel tissue with time. In addition, domoic acid concentrations in mussels from two infected rivers declined to negligible levels in 40-50 days under normal environmental conditions. The bulk of domoic acid and toxicity was located in the hepatopancreas which also contained large amounts of chlorophyll-A, an algae biomass indicator, relative to control mussels. These results support the conclusion that domoic acid was the primary causative factor in the shellfish poisonings from Prince Edward Island mussels in late 1987.