The provision of animal feed pellets is described, which pellets comprise, as antibiotic, a pleuromutilin derivative is stabilized form, namely in the form of microspherules. The pleuromutilin derivatives in question have the general formula (I) wherein R1 is ethyl or vinyl, there is either a double bond or a single bond between carbon atoms 1 and 2, Ra and Rb are each independently of the other hydrogen or halogen, and T is a short or long-chain organic radical.