There are striking age-related changes in the demography of thymus lymphocytes, i.e. in thymus-cell subpopulations of BALB/c and SJL mice; these changes occur in the proportion of cells, identified by various markers, and by the membrane density of these markers. The thymuses of both strains undergo an age-related increase in the proportion of CD4+ CD8- cells and decrease in CD4+ CD8+ cells. Age-related changes in cells that are Pgp-1+ also show marked strain differences: Pgp-1+ cells increase in SJL, but not in BALB/c thymuses. In both strains, cells with high density of Pgp-1 appear in later life, though this is more marked in the thymus of SJL, which also shows a higher relative density at an advanced age, than do BALB/c mice. Furthermore, the per cent of cells with high density of Pgp-1 is larger in thymuses of SJL than in BALB/c mice. The percentage of CD45+ thymocytes remains unchanged, as animals age. Thymocyte-membrane densities of CD-45 undergo age-related increases in both SJL and BALB/c. The per cent of cells with high density of CD-45 is similar in both strains. Individual variations in relative size of subpopulations in SJL mice of the same age are greater in old than in young mice; this increase in heterogeneity is manifested by increase in standard deviation. Corresponding significant changes have not been observed in BALB/c or C57BL/6 mice. Thus, we have detected an intrastrain variation which may reflect age-related effects of the impact of stochastic events.