Cell-mediated immunity to Besnoitia jellisoni (protozoa) was studied in golden hamsters (Mesocricetus auratus) by total-body irradiation. The LD50 for hamsters was 900–925 röntgens (r). Active immunization was inhibited by 600 r given from 22 days before to 28 days after infection. Antibody production was markedly impaired only by 600 r given 4 or 24 hr after infection. After immunity had developed, fatal relapse occurred sporadically after supralethal, divided doses of up to 1,800 r. Lymphoid cells transferring adoptive immunity were destroyed in donors by 50–100 r. Allogenic recipients accepted lymphoid cells after a dose of 600 r, but were no longer capable of benefiting from adoptive immunization after receiving 800 r. Neither radiation that destroyed adoptive immunization (100 r) nor supralethal radiation (1,600 r) released demonstrable cell-immunity factors into the plasma or serum. Transplantation requirements for adoptive immunization against besnoitiosis could only be fulfilled by isogeneic or irradiated allogeneic recipients. Conversely, transplantation requirements could be neglected in protection of allogeneic hamsters against infection with equine rhinopneumonitis virus, in which immunity develops in two to four days. The radiation sensitivity and other characteristics of besnoitia infection are compared with listeriosis, another model of cellular immunity.