Abstract: Many attempts to demonstrate the dietary essentiality of fluoride have yielded equivocal results and none of the studies have connected fluoride to a specific metabolic deficiency (refs. 1-5 and personal communication from K. Schwartz). Undoubtedly, these studies have been hampered by the ubiquitous occurrence of fluoride in nature foods and water with the result that it has been very difficult to prepare a diet devoid of fluoride but nutritionally adequate in all other respects. In the work now reported, we took advantage of the low fluoride contents of milk and cereals to prepare a diet of very low fluoride content compared with that of commonly used rodent foods. Through use we found that haemopoiesis is markedly disturbed and the addition of fluoride to the intakes of the animals prevented this abnormality.