The lifespan of T lymphocytes is of particular interest because of their central role in immunological memory. Is the recall of a vaccination or early infection, which may be demonstrated clinically up to 50 years after antigen exposure, retained by a long-lived cell, or its progeny? Using the observation that T lymphocyte expression of isoforms of CD45 corresponds with their ability to respond to recall antigens, we have investigated the lifespan of both CD45RO (the subset containing responders, or ‘memory’ cells) and CD45RA (the unresponsive, or ‘naive’ subset) lymphocytes in a group of patients after radiotherapy (Michie et al. 1992). We have found a rapid loss of unstable chromosomes (which result in cell death in mitosis) from the CD45RO but not the CD45RA pool. Immunological memory therefore apparently resides in a population with a more rapid rate of division. The survival curves for the two populations are best described by a model in which there is also reversion in vivo from the CD45RO to the CD45RA phenotype. Expression of CD45RO in T cells may therefore be reversible. Further data showing survival curves of T lymphocytes with stable radiation damage (passed to one daughter cell during mitosis) is also considered. These curves show very little loss of such cells. The difference between the two populations (stable and unstable damage) allows an estimate of their proliferation rates and death rates. These parameter estimates may be of interest to people modelling the dynamics of the immune response as they give some rough indicators of the timescales on which T lymphocytes turn over (McLean and Michie 1993).