The response of thymic cells to ConA and PHA was followed during 49 days in 9 Gy-irradiated C3H mice reconstituted with (C3H X AKR)F1 BM. Thymic suspension were fractionated firstly according to their capacity to bind PNA, and secondly by their Thy-1 surface antigen: Thy-1.1 antiserum was used either to lyse doner-derived cells or to separate them from host cells by panning. From days 14 to 25, when the donor-derived elements expand rapidly, the PNA- fraction remains stable and equal to 6%, suggesting that PNA+ and PNA- cell populations develop independently of each other. On the contrary, the PNA- fraction which initially represents 6% of the surviving cells rises up to 12% after day 20 and remains at this level in the long-lived host population until the end of the experiment (49 days). The response to ConA returns to normal as early as 15 days after X-rays in the PNA- fraction, whereas the response to PHA is still impaired at the end of the experiment. In both cases host cells express a higher level of responsiveness to mitogens than donor cells.