AbstractThe role of spleen and thymus in maturation processes of B cell subpopulations was examined using mice which underwent fractionated total lymphoid irradiation (TLI). (BALB/BL)F1mice received 200 rds lymphoid irradiation daily for 8 days. Mice were spleen‐shielded or ‐unshielded, or thymus‐shielded or ‐unshielded during the irradiation. One day after termination of the irradiation, TLI‐treated unshielded mice were reconstituted with 10 × 106spleen, thymus or bone marrow cells of normal untreated syngeneic mice. Two and a half months after termination of the treatment, the ability of the treated mice to producein vivoanti‐trinitrophenyl (TNP)‐Ficoll antibodies, and the capability of spleen cells of those mice to respondin vitroto dextran sulfate and lipopolysaccharide was checked. In parallel, stained spleen cells were analyzed on the fluorescence‐activated cell sorter. The results indicate that B cell maturation occurs only in mice where the spleens were shielded during TLI‐treatment or in TLI‐treated mice reconstituted with spleen cells of normal untreated mice. In these mice, the light scatter and the fluorescence distribution profiles were the same as those obtained from spleen cells of control mice: the treated mice gave a high anti‐TNP‐Ficoll antibody response and the proliferative response of the cells was low to dextran sulfate and high to lipopolysaccharide.Thymus shielding during TLI treatment or reconstitution of TLI‐treated mice with thymus or bone marrow cells could not abolish the blockage of B cell maturation processes. These findings indicate that the spleen plays an obligatory role in B cell subset maturation pathways, whereas the thymus appears to play no essenti