The cellular composition of the thymus was investigated as a function of age in the immunologically aberrant SJL/J mouse strain. Lymphocyte subpopulations were identified by combined analysis of cell electrophoretic mobility (EPM) and cell electronic volume and by assessment of surface receptor for Peanut-agglutinin (PNA) and of surface immunoglobulin (slg) determinants. In the thymus from young adult animals two major types of thymocytes could thus be recognized. The first one (th1, 2, 3) representing about 75% of the thymocyte population was endowed with a low-EPM and exhibited PNA-receptors. The other one (th4) possessed a high-EPM and lacked PNA-receptors. During ageing of mice selected for the absence of macroscopically detectable Reticulum Cell Sarcoma (RCS) lesions, the frequency of th1, 2, 3 cells diminished whereas that of th4 cells increased (up to 75% at the age of 16 months). This latter augmentation reflected a true expansion of the th4 cell subpopulation and acounted for the maintenance of thymus cellularity to a relatively high level throughout life. In accordance with the probable immunocompetence of th4 cells, thymus cell suspensions from old RCS-free SJL/J mice were found to exhibit high proliferative responses to T-cell mitogens. On the other hand, from the age of 8 months onwards, a new physical type of lymphocytes (th5) could be detected in increasing proportions. These cells were characterized by a lower EPM than typical th1, 2, 3 thymocytes and by a modal volume around 150 micrometer3. They were further demonstrated to be PNA- but slg+ and are thus likely to represent B cells. Such alterations were not encountered in BALB/c and DBA/2 mice in which both th1, 2, 3 and th4 thymocyte subpopulations regressed at approximately the same rate with age. Moreover, in the thymus of RCS-bearing SJL/J mice, the hyperplasia of the th4 cell pool and the occurrence of th5 cells appeared less important than in RCS-free mice.