Antigen-specific suppressor factor produced by metabolically active in vitro-induced suppressor cells, upon further antigenic stimulation, act on nylon wool nonadherent, Ly-2-negative target cells within helper cell population, resulting in suppression of both the IgM and IgG antibody responses. Thus the target is an Ly-1+ T cell, possibly the helper cell. All the mouse strains tested so far have been able to produce the factor, and when tested in CBA or B10 mice, there seems to be no genetic restriction involved e.g., nonsyngeneic suppressor factors suppress as well as do the syngeneic factors. Comparison of the properties of suppressor factor with those of extracts of suppressor cells yield differences in origin, target of action and effect, indicating that these are different molecules. The heterogeneity of suppressor pathways is discussed.

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