Survival data were obtained on Acholeplasma laidlawii B after exposure to ionizing radiation at various ages, pH, temperature, and supplements to the medium. The D0 value varied slightly with pH in air but much more after irradiations in N2 or N2O. Replacement of the growth medium with fresh broth resulted in an increase in D0 while dilution of the growth medium with Tris-NaCl 1:100 caused a decrease in D0 both in air and N2. The D0 varied with the age of the culture and the temperature at the time of irradiation. In air the ratio of increment in D0 (ΔD0) over the increment in temperature (ΔT) had positive values as a function of age of the cells while under N2 this ratio was always negative. In N2 the D0 decreased in a smoothly non-linear fashion with an increase in temperature while in air it remained constant below 12° and increased linearly above that temperature at a rate equivalent to 1.65 kcal/mol. ATP was found to radioprotect the cells in N2 at pH 6.3 but radiosensitize them both in air and N2 at pH 7.5. The adenine nucleotides ATP, ADP, and AMP eliminated radiation-induced division delay in the cells in air. Cyclic AMP had no effect on the division delay in air but was found to synchronize cell growth.