The in vivo helper function of specific thymic inducer cells in anti-sheep hemolytic responses was retained after exposure to 1000 rads of γ-rays, but not after 3000 rads. A prerequisite for B-T cell interactions involving irradiated T cells was the residence (i.e., differentiation) of bone marrow-derived precursor cells outside the bone cavities, e.g., in the spleens of irradiated nonthymectomized mice. It is suggested that reproductive viability of T cells is not essential for inducing γM or γG responses in vivo, and that the properties acquired by B cells in the splenic environment confer susceptibility to the effects of nonantigen-specific T-cell mediators.

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Research supported by United States Public Health Service Grants AM-13,969 and CA-12,844.

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