The clastogenic potential of sodium fluoride was determined both in vitro (using cultured human lymphocytes) and in vivo (using the rat bone-marrow micronucleus test). The incidence of chromosome aberrations in human lymphocyte cultures exposed to 20 or 40 micrograms/ml sodium fluoride (3 and 9% respectively) was significantly increased compared with control cultures (0.5%). However, the incidence of micronucleated polychromatic erythrocytes in male AP rats dosed 1000 mg/kg NaF (the maximum tolerated dose over 24 h) or 500 mg/kg NaF was similar to that in the animals dosed distilled water (vehicle control). Thus, sodium fluoride is clastogenic in vitro but not in vivo.