The activity of suppressor cells from spleens of mice of varying ages was assessed by their addition to cultures of normal or SRBC immune spleen cells together with a challenge of SRBC. 1-wk and adult spleen cells were highly suppressive of the secondary in vitro antibody response to SRBC. 3-wk spleen cells were less active in suppressing this response. The nature of the suppression and the character of the suppressor cells changed in this period. Whereas adult spleen cells demonstrated specificity, 1-wk cells nonspecifically suppressed all responses tested. Further, unlike adult suppressor cells (which are Thy.1.2 positive), 1-wk suppressor cells are insensitive to anti-Thy.1.2 treatment in this system. Both cells are nonadherent to glass beads and nylon wool and are undetectable in the normal thymus.

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