Polyclonal activation of murine splenic B lymphocytes by lipopolysaccharide was found to be subject to regulation by helper and suppressor influences from T lymphocytes. In the normal adult spleen, only helper influences were exercised over polyclonal B cell activation; this influence is a property of Lyt-l(+)23(-) slowly sedimenting T cells. Suppressive influence evidently is latent, for it exists at such a low level (or the cells are so few in number) that its effects are difficult to detect. Suppressor T cell function may be evoked by culturing spleen cells at high ratios of T:B cells, by activating splenic T cells with concanavalin A, or by sonicating unstimulated splenic T cells to liberate a suppressive potential that is not expressed by these unstimulated cells when intact. The soluble fraction of resident splenic T cell sonicates exerts both helper and suppressor regulatory influences. The soluble helper activity is derived from Lyt-l(+)23(-) slowly sedimenting T cells, whereas suppressor activity is generated from a distinct subpopulation of Lyt-l(-)23(+) rapidly sedimenting T cells. The thymus contains cells capable only of helping but not of suppressing polyclonal activation of splenic B cells. Helper and suppressor activities contained in splenic T cell sonicates were separated by gel chromatography; the suppressive activity was found to elute with a molecular weight between 68,000 and 84,000 and the helper activity eluted with a molecular weight between 15,000 and 23,000. The data indicate that helper and suppressor activities are distinct molecular entities derived from distinct splenic T lymphocyte subpopulations. The possibility that these molecules are precursors to or components of antigen- specific or nonspecific helper and suppressor factors described in the literature is discussed.