Chromatographically purified ciguatoxin from toxic blackfin snapper (Lutjanus buccanella) obtained from an impounded catch in the U.S. Virgin Islands was used together with a refined mouse bioassay to construct a dose-response curve. The LD50 was calculated to be 87 mg/kg and the range of ciguatoxin doses between 0% and 100% lethality was narrow (62.5-112.5 mg/kg). Symptoms appeared in a rather stereotypic pattern with the most frequent being lowered rectal temperature and reduced locomotor activity and reflexes. Diarrhea and cyanosis were common. Breathing difficulties and convulsions occurred more often at higher doses. Approximately one-half of the animals which succumbed to ciguatoxin showed convulsions at some time, often just prior to death. The mouse bioassay described permits a reproducible and moderately sensitive detection of ciguatoxicity and would, if uniformly adopted, facilitate a comparison among future reports on ciguatera research.