The production of congenic lines of mice differing at immuno-globulin loci is described. After five backcross generations a pair of lines was developed which, although skin-compatible, still have barriers for transferred spleen cells, as followed by quantitating donor type γ-globulins in the recipients' sera. The barriers are radiation-sensitive and can be overcome by large numbers of transferred cells. One barrier, which can be largely overcome by 300R to the recipient is described as lack of “biological space.” The second barrier may be residual histoincompatibility. Spleen, bone marrow, and fetal liver but not thymus cells produce γ-globulins for many weeks after injection into 600R-irradiated congenic recipients. As donor type levels increased, host type decreased in some animals to very low levels. The usefulness of the congenic strains being produced for studies of cell immunological potential, regulation, and function is discussed.
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