Abstract
Lymphoid cells of bursa of Fabricius taken from 2 1/2‐week old chickens were separated in a discontinuous albumin gradient into six fractions. At this age, the bursa contains bursal stem cells and early postbursal cells; only the bursal stem cells are capable of restoring the bursal morphology of cyclophosphamide‐treated recipients. Cells from each fraction of the albumin gradient were injected into 4‐day old cyclophosphamide‐treated, histocompatible recipients. Forty days after the cell transplantation, microscopic morphology of bursa and spleen, and antibody formation to sheep red blood cells and Brucella abortus were studied. The low density bursa cells were found capable of a significant restoration of the bursal morphology, while those with a higher density had significantly less effect on the bursal morphology. When bursa cells from 10‐week old donors were used, no clear fractionation was obtained according to the capacity to restore the bursal morphology of cyclophosphamide‐treated recipients. These findings indicate that the bursal stem cells capable of restoring bursal structure of cyclophosphamide‐treated chicks are, in average, lighter in density than the postbursal cells.