A factorial experiment was conducted with weanling rats fed a purified diet to determine the influence of dietary protein type (casein or lactalbumin) and level (120 or 360 g/kg) on fluoride bioavailability (2 or 10 mg/kg as NaF). Although total food intake during the 4-wk study was similar for all eight treatment groups, rats fed the high protein--containing diets gained significantly (P less than 0.001) more weight than did rats fed the low protein--containing diets. Rats fed high protein diets absorbed significantly (P less than 0.001) more fluoride than rats fed low protein diets at either fluoride level regardless of protein type. The fraction of absorbed fluoride that was actually retained, however, was significantly (P less than 0.001) reduced by high protein at either fluoride level regardless of protein type. This depressive effect of high dietary protein on retained fluoride was reflected in femur fluoride concentration. Our results suggest that food fluoride, such as that originating from preparation of foods with fluoridated water, will be less bioavailable for an individual ingesting a high protein diet than for an individual fed a low protein diet.