Summary: Rabbits irradiated with 450 r 1 day previously, that received both rabbit peritoneal macrophages incubated with bovine gamma globulin (BGG) and nonimmune lymph node cells from the macrophage donors, were able to produce antibody. Rabbits similarly irradiated and injected with sensitized macrophages alone failed to show an antibody response. Normal recipients of these cells exhibited a typical primary antibody response, although titers were lower than in controls receiving antigen alone. Normal and irradiated rabbits receiving lymph node cells incubated with antigen also failed to produce antibody, as did irradiated rabbits receiving simultaneous injection of antigen and lymph node cells. Peritoneal exudate cells were harvested from rabbits which had received 450, 550 or 750 r x-irradiation 24 hr previously. After incubation with BGG these cells were injected into normal rabbits. When donors had received 750 r, no antibody response was elicited in the recipients. The response to sensitized irradiated macrophages which had received 450 or 550 r was variable. In no case did the degree of antibody response appear to depend on the number of sensitized cells injected. Irradiated recipients receiving sensitized irradiated macrophages (550 r) together with lymph node cells from normal rabbits were unable to form antibody. The implications of these findings in relation to the radiosensitive phase of the antibody response are discussed.