The capacity of the blood lymphocyte population to spontaneously lyse K562 and Chang cells In vitro was examined in women who received local radiation therapy (45.0 Gy) for breast cancer. It was observed that cytotoxicity, on a cell-for-cell basis, was significantly reduced against K562 cells at completion of irradiation. This was followed by a recovery to the pretreatment level within 3-4 months and remained relatively constant for approximately 2 yr. The pattern of these changes were reasonably similar to that of the frequency of lymphocytes expressing Fc-receptors for IgG. In contrast, the relative cytotoxicity against Chang cells was unchanged at completion of irradiation. At 3-4 months, however, the relative cytotoxicity was increased above the pretreatment level and remained elevated for at least 2 yr. The results indicate that different effector cells are involved in the spontaneous destruction of K562 and Chang cells.