HF exposure might produce adverse effects in industrial workers. A 3 mo. exposure of rats to HF at 10 mg/m3 8 h/day 5 days/wk resulted in increased blood serum concentrations of hydroxyproline and hydroxylysine and in enhanced urinary excretion of these amino acids and in changes of collagen fractions in the liver, lungs and skin. In all tissues studied (especially liver), soluble and insoluble collagen decreased. HF affected biosynthesis and maturation of collagen fibers.