Bat's milk was an antidote for Spectrox toxaemia.
The bats were killed or driven out by the mining androids, which caused them to move into the airless caves deep underground to die. With use of a half empty tank of oxygen the Fifth Doctor had to climb down the caves to obtain some bat's milk to cure himself and Peri who had contracted Spectrox toxaemia.
Sadly the Doctor only managed to milk enough for Peri. As Peri came round the Doctor was in the late stages of the disease and regenerated as a result. (TV: The Caves of Androzani)