The spontaneous cytotoxicity of blood lymphocytes was examined before and after local radiation therapy (64 Gy) for transitional cell carcinoma of the bladder. The lymphocyte count was reduced to approximately 40 per cent at termination of treatment and remained below the pre-treatment level during a follow-up period of 18 months. The relative spontaneous cytotoxicity for 51Cr-labelled K562 cells, derived from a human myeloid leukemia, increased at completion of irradiation and remained elevated during the entire observation period. The relative cytotoxicity against Chang cells, derived from human liver, also seemed to increase but not to the same extent. The increments were probably due to an increased proportion of spontaneously cytotoxic cells bearing Fc-receptors for IgG which were significantly elevated after treatment. It was concluded that the changes differ from those occurring after local irradiation for breast carcinoma. The proportion of lymphoid tissue in relation to the blood volume which is included in the irradiated field may determine the changes of the blood lymphocyte population.