Fluorine is determined in the ash of bone or tooth tissue after the sample has been dissolved in dilute perchloric acid. Interfering phosphate is precipitated by the addition of silver perchlorate and sodium carbonate and then filtered off. The fluoride contained in the filtrate is determined spectrophotometrically, by the bleaching of the coloured complex of zirconium with Xylenol Orange. The reaction is carried out in dilute perchloric acid and allowed to proceed for 1 hr. The absorbance is measured at 540 mmu. The relationship between the absorbance and concentration is practically linear over the range 15-65 mug. The range can also be modified for 0-50 mug.