The effects of radiation upon circulating lymphocytes have been measured by two independent means, one monitoring the level of cytogenetic damage appearing in these cells, the other monitoring changes in their proliferative response to stimulation in culture by phytohaemagglutin (PHA). Response to PHA can be expressed quantitatively as lymphocyte replicating ability (LRA), i.e. the degree of ³H-thymidine uptake measured under standardized conditions. Varying degrees of depression of LRA have been noted within a group of patients undergoing a wide range of radiotherapeutic procedures. The concomitant cytogenetic studies indicate that although cells bearing radiation damage do persist in the circulation after irradiation their numbers are insufficient to account for the marked reduction seen at the same time in LRA. It appears that the reduction is largely due to a fall in the number of thymusdependent lymphocytes in the circulatory pool.