During our studies on the progenitor or stem cell responsible for humoral immunity (1, 2) it became evident that the organ distribution of the stem cell does not parallel the distribution of immunocompetent cells. In the present work, we have studied the occurrence of immunocompetent cells in different chicken tissues during different stages of ontogeny. For this purpose, immunoeompetent cells for humoral immunity are considered those cells which react with antigens and produce antibodies, without defining how many cell types are involved.Previously, different methods have been applied to studies of the immunocompetent cells in avian tissues. Studies by several investigators (3-5) have demonstrated that spleen cells of immunized adult and young chickens adhere and lyse sheep red blood cells (SRBC), ~ whereas cells taken from bursa of Fabricius are inactive in this respect. The earliest studies on antibody response by transferred cells suffered from allogeneic rejection (6, 7). To avoid such rejection, newly hatched and embryonic recipients have been used to demonstrate antibody production by spleen, bone marrow, and thymus cells from immunized and normal donors (8)(9)(10)(11)(12)(13)(14)(15)(16). In these models, however, the transplanted cells induced a graft-vs.-host reaction in the young recipients. Graft-vs.-host reactions were also obtained when hormonally bursectomized recipients were used (13,17). In surgically bursectomized chickens, bursal transplants, as such or in a diffusion chamber, were found to increase antibody synthesis if the chickens were stimulated within 3 days after transplantation (18-21).The histoincompatibility reactions can be mostly avoided by having birds isogeneic at the B locus. Using this model, Orlans and Rose (22) showed that preimmunized