Tiamulin is a semi-synthetic derivative of Pleuromutilin. It is highly active against gram-positive organisms such as streptococci, staphylococci and against mycoplasmas and serpulina (Treponema) hyodysenteriae. A number of Shigella, Klebsiella and E. coli are also quite susceptible to this agent. Some anaerobic species including Bacteroides fragilis and Clostridium perfringens with no exception are sensitive to tiamulin.

Tiamulin acts bacteriostatically. The biochemical basis for the antibacterial action is inhibition of protein synthesis, by binding to 70S ribosomes.