E.Coli, YC-1 strain, were subjected to 40-C irradiation under various conditions and the survival ratios measured. The following results were obtained: With radiation dosage held constant, prolonged irradiation produced less bactericidal effect. Temperature and environment before irradiation had little influence on the bactericidal effect. Preirradiation washing had little effect. In the earlier stage metabolism continued but gradually decreased later. The more densely the suspension is populated, the higher the survival ratio. This ratio is decreased when a large number of bacteria is treated with a sublethal dose of streptomycin before irradiation. Treatment with chloramphenicol showed no effect in a small population, but the survival ratio decreased in a large population. Penicillin caused a decrease in a large population also, but in a small population the ratio became markedly high. It is presumed that the activity of bacterial metabolism is closely related to the bacterial action of irradiation. (P.C.H.)