When thymus cells which are unresponsive to LPS are combined with numbers of peripheral lymphoid cells giving minimal responses to LPS, synergistic incorporation of [3H]thymidine occurs. Synergy requires that both components proliferate, but most of the augmented response is the result of peripheral cell proliferation. The thymus cell is a T cell of variable density, low in thy-1.2 antigen, not concanavalin A responsive, present in the major thymus subpopulation, and may be from lipopolysaccharide (LPS)-unresponsive strains. The peripheral cell is sensitive to anti-IgG or IgM plus complement (C'), resistant to anti-Thy-1.2 and C', exhibits adherence properties of B lymphocytes, and must be from LPS-responsive strains. Synergistic responses depend on critical thymus/peripheral cell ratios, inhibition occurring at high peripheral cell numbers. The data provide evidence that B-cell proliferative responses to LPS may be regulated by a subclass of thymus T cells.

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