Viable cultures, culture supernatants, and killed sonicated preparations of various species of arginine-degrading mycoplasmas are capable of inhibiting cell division in L5178Y cells. The inhibition can be prevented by adding excess arginine at the same time as the mycoplasmal material and can be reversed by adding arginine after the cells have been blocked in division. Partially purified arginine deiminase from Mycoplasma orale and from baker's yeast show similar toxicities. The mycoplasmal enzyme is inhibited by specific antiserum. When L5178Y cells are cultured in arginine-free medium, cell division stops, but the inhibition can be overcome by the addition of arginine.