Phytohemagglutinin-induced T cell colony ( > 50 cells/aggregate) and cluster (10–50 cells/aggregate) formation was studied by one-stage microagar culture in glass capillaries by peripheral blood mononuclear cells from 26 young healthy individuals (20–32 years) and 26 aging subjects ( > 65 years). The number of colonies developed after phytohemagglutinin (PHA) stimulation of mononuclear cells from aging subjects was significantly increased (p < 0.05) when compared to those from simultaneously studied young individuals. Increase in the number of PHA-induced colonies by lymphocytes from aging subjects over that of young subjects was observed at different concentrations of PHA. The highest level of colony formation in mononuclear cells from both aging and young subjects was obtained with 10 μg/ml of PHA. Possible mechanisms for increased T cell colony formation in aging humans are discu