To discover whether individual bursal follicles can contain clones of B lymphocytes, we estimated the numbers of lymphoid cell precursors populating single follicles in two types of chicken chimera. The first type was produced by establishing parabiotic connections between blood vessels of embryo chorioallantoic membranes. Under these conditions, and most likely during normal development, most follicles are populated by more than one, but less than ten, precursor cells. However, in a second type of chimera, a cyclophosphamide-treated chick reconstituted with normal bursal cells, most follicles in the reconstituted bursa are clonal (their lymphocytes are derived from a single precursor cell). Individual follicles can readily be isolated from bursae of reconstituted birds and should be useful in studies of B cell development.