The effects of the inhalation of hydrogen-fluoride (7664-39-3) (HF) on several animals was investigated. Five rabbits, three guinea-pigs and two Rhesus-monkeys were exposed to atmospheric HF at a concentration of 0.0152 milligrams per liter for 6 hours a day. This was repeated for 30 days until a total of 309 exposure hours were accumulated. Blood counts were made biweekly for the first 25 days and weekly for the remainder of the experiment. All animals were weighed daily during the exposure period and weekly thereafter. There was no pronounced response following introduction of the animals into the exposure chamber. After 160 hours of exposure, one guinea-pig began to lose weight. Exposure was discontinued but he died 3 weeks later. After 134 hours of exposure, a second guinea-pig also began to lose weight and eventually died. All other animals survived the exposure period. One rabbit became pregnant and gave birth to three normal young 13 days after conclusion of the exposure period. The exposed group had a lower erythrocyte count compared to controls. The mean values were 5323 and 5657, respectively. Necropsies on the exposed group showed damage to the lungs, livers and kidneys in rabbits and guinea-pigs; however, except for the kidneys, the organs of the monkeys showed scarcely any lesions. The authors conclude that this experiment demonstrates the noxious properties of atmospheric HF at a concentration where work is possible and an immediate physiological response is lacking.