The investigation of a high incidence of arthritis in 21 dairy herds disclosed elevated fluorine levels in bone samples. In every herd except one, where herbage and water was contaminated by industrial fall-out, the main source of the fluorine was from mineral supplements; In a few herds, purchased cake or grain balancers contributed to the abnormal levels. Over 100 cows with arthritis had fluroine levels in the bone of between 2000 and 8000 ppm, or were in herds whose diet contained excess fluorine. Characteristic tooth lesions often confirmed the link between arthritis and fluorosis. Sixteen out of 31 samples of mineral supplement contained dangerous levels of fluorine (3000 to 13000 ppm). Grain balancers contained up to 400 ppm F, and dairy cake had levels as high as 140 ppm F. There was a statistical correlation between a high incidence of damage to peri-articular structures, resulting in debility and loss of production, and elevated bone fluorine.