F1 hybrid chicks isogenic for the strong B histocompatibility locus and for most weak H-loci were X-irradiated on day 1 after hatching, injected intraperitoneally on day 2 with dispersed cells of bursa, spleen, or thymus from 4- or 10-wk-old F1 hybrid donors, and immediately challenged by the same route with either Brucella abortus, sheep erythrocytes, or a mixture of both together. The agglutinin titers were measured in sera obtained 1 wk later. With 4-wk-old donors, a greater primary response to Brucella abortus was obtained after transfers of cells from bursa than from spleen, while thymus was much less effective. With 10-wk-old donors, the decreasing order of response was spleen, bursa, thymus. Only splenic cells were effective in transferring a response to sheep erythrocytes, at either donor age. In tests of synergism by cell mixtures from pairs of organs, the only positive finding was a modest augmentation of titer against sheep erythrocytes by bursa + spleen as compared with spleen alone. Bursal cells from 6- or 10-wk-old donors were effective in transferring a response to sheep erythrocytes when antigen injection was delayed until 5 days after cell transfer. Splenic cells from hormonally bursectomized donors were ineffective in transferring a primary response, even when the donors had been injected with antigen 1 wk before transfer. Preimmunization of normal donors led to marked increases in the responses to Brucella abortus produced by transferred splenic or thymic cells. With bursal cells, an increased response was obtained only if the interval between preimmunization and transfer was 17 rather than 7 days. With the 17-day interval, both bursal and thymic cells could also transfer a response to sheep erythrocytes. The primary sera to Brucella abortus produced after transfers of bursal or splenic cells contained almost entirely 19S antibodies. A 7S component was found in all the secondary sera tested.