Three-day-old cyclophosphamide-treated chicks were transplanted with cells from the bursa of Fabricius of age-matched histocompatible donors. After the cell transplantation (0–41 days) the birds wore killed, and the bursas were removed, weighed, and studied histologically. Cyclophosphamide treatment caused a practically total depletion of lymphoid cells from the bursal follicles. In chickens that received bursal stem cell transplants after cyclophosphamide treatment, a gradual restoration of bursal follicles was observed. First signs of the repopulation of the bursa by lymphoid cells were collections of large and medium sized pyroninophilic cells seen in the follicular epithelium and in the lamina propria already on the day after the transplantation. On the 3rd and 6th days, the bursal follicles were in different phases of their redevelopment containing varying numbers of large and medium sized pyroninophilic cells and a few small lymphocytes. Proliferation of these cells led to an independent redevelopment of each of the bursal follicles. From the 6th day onward, the number of well developed follicles increased gradually together with the relative weight of the bursa. Twenty-seven and 41 days after the transplantation, the histological appearance of the bursa was similar to that, found in untreated normal animals. Repopulation of the cortical zone in the bursal follicles occurred only after the medulla, i.e., the follicular epithelium. was first repopulated by lymphoid cells. This order of development, is the same as found normally in the bursa during the ontogeny.