The blood and bone marrow of New Hampshire chicks were analyzed quantitatively from the time of hatch to 8 weeks of age. Hormonal bursectomy was performed by treating embryonating eggs on the 11th day of incubation with testosterone propionate (TP) which resulted in severe hypogammaglobulinemia and complete atrophy of the bursa of Fabricius. TP-treated birds exhibited some lymphocytopenia, reduced splenic weight, and lack of plasma cells in their bone marrow. The number of cells per milligram bone marrow was comparable in normal and TP-treated birds, falling in the range reported for laboratory rodents. The chick medullary hemopoiesis is characterized by the predominance of erythroblasts throughout early development; granulocytes and lymphocytes represent much smaller cellular compartments than in rodents. In the chick granulocytes tend to decrease after hatch, whereas in rodents they tend to increase. The normal chick shows a temporary increase in marrow lymphocytes after hatch, similar to that observed in some young rodents, but on a much smaller scale. Hormonal bursectomy did not prevent the development of a lymphocyte population in the bone marrow. These cells were fewer in TP-treated birds at hatch and at 4 weeks than in normal birds, but at 8 weeks of age normal and bursectomized chicks had comparable numbers of lymphocytes in their marrow. Although some lymphocytes in avain bone marrow may depend on the bursa of Fabricius for their development, a substantial proportion of bone marrow lymphocytes in the chick are bursa independent. The cell surface phenotype and site of origin of these cells remains to be investigated.