Calves were given daily extracorporeal irradiation of circulating blood (ECIB), continuous extracorporeal irradiation of thoracic duct lymph (ECIL), and thoracic duct cell depletion by continuous centrifugation to determine the effects of lymphocyte depletion on primary and secondary antibody responses to tetanus toxoid. These procedures induced a severe depletion of small lymphocytes in circulating blood and thoracic duct lymph. Although the appearance of detectable serum antibody was delayed 5 to 10 days and peak titers were slightly reduced, all calves depleted of circulating lymphocytes gave nearly normal primary tetanus antitoxin responses. Repetitive ECIB to 250 to 860 blood volumes during a period of 10 to 30 days with accumulated radiation doses of 93,000 to 250,000 rads failed to significantly repress secondary responses to fluid tetanus toxoid. These findings are in sharp contrast to the well-known radio-sensitivity of antibody responses when total doses of 50 to 600 rads of ionizing radiation are delivered by whole-body exposure prior to immunization.

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