Embryonic and newly hatched chicken splenic lymphocytes were analyzed for their ability to respond to 2 mitogens, Con A and PHA. Acquisitions of the ability to respond to both mitogens began at hatch and responses to both mitogens reached normal adult blastogenic response levels by 11 days post-hatch. Splenic lymphocytes in the developing embryo and newly hatched chicken were found to be capable of inhibiting the Con A- and PHA-mediated blastogenic responses of both syngeneic and allogeneic

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... t splenic lymphocytes. The ability to suppress adult responses was first observed on E12, reached a maximum of suppressive ability on E18, and the suppressive ability was gradually lost with development such that no suppressive ability was exhibited after H11. Ability to suppress adult splenic lymphocyte proliferation to mitogens was found to not be a unique property of splenic lymphocytes but was also exhibited by bone marrow cells. The ability of bone marrow cells to suppress adult splenic lymphocyte proliferation to Con A and PHA gradually decreased with chicken aging but the suppressive ability was never lost and adult bone marrow cells were found to exhibit significant degrees of suppressive ability. The mechanism of suppression by the 1-day-hatched splenic lymphocytes was found to be cell-to-cell contact mediated and did not require any newly synthesized H1 splenic products, since x-ray killed cells were able to suppress as well as live cells.