There are two types of RFC in lymphoid cells of normal unimmunized chicks, which form rosettes with QRBC. They might be separable subpopulations of lymphoid cells in reference to the thymus dependency. Ontogenetic developments of both of the RFCs suggest the presence of their interrelations in the maturation process of the lymphoid system in chicks. The nature of the receptor site for QRBC and the source or origin of RFCs are discussed.