Fify-seven Huichol Indians with a lifelong individual history and a 1,600-year cultural tradition of ingestion of peyote, a mescaline-containing cactus possessing hallucinogenic properties, were compared with 50 Huichol Indian controls and ten laboratory controls for effects on lymphocyte chromosomes. The frequency of abnormalities in the experimental and control groups did not differ significantly. Our results indicate that multigenerational ingestion of peyote is not associated with abnormalities in lymphocyte chromosomes.