The frequency of antigen responsive cells (ARC) specific for mouse red blood cells (MRBC), as measured with the cell transfer assay, was consistently high in the spleen and blood of MRBC-immunized chickens but negligible in unprimed juvenile birds. Practically all of the adoptive antibody formation in the host embryos could be attributed to the presence of ARC's in the cell suspensions from immunized birds and very little to passively acquired donor plaque-forming cells (PFC). When the spleen and blood cells from unprimed donors were retested in the in vivo immune assay, as found before, the host embryos failed to respond to a single MRBC exposure given at the time of cell transfer. However, substantial splenic PFC formation occurred in response to a second MRBC injection given four days later. In addition to the spleen, the peripheral blood of the unprimed and antigen-primed chicken represents a significant reservoir of immunocyte precursors.