A single thymus lobe from a neonatal donor has the capacity to restore primary immunological competence to a thymectomized syngeneic radiation chimaera. If a cytologically marked thymus graft is used the fate of the introduced cells following antigenic stimulation may be studied. Thymus-derived cells can be found in the spleen and lymph-nodes of such mice and furthermore there is a transient but well-defined increase in the proportion of these cells dividing following an injection of sheep red blood cells or application of a skin homograft 30 days after irradiation and thymus grafting. It was found that an even greater proportion of thymus-derived cells divide in response to a second challenge with sheep cells. Similarly, a higher percentage of thymus-derived cells divide in the lymph-nodes draining a second skin homograft.