The influence of age on the number of receptors for insulin and glucocorticoids on human T cells was examined. The specific binding of 125I-insulin and 3H-dexamethasone to phytohemagglutinin-(PHA)-stimulated T cells was found to decrease with age. Populations of PHA-stimulated T cells, however, are heterogeneous with respect to cell cycle phases, and cells in different cell cycle phases have been shown to bear different numbers of receptors. Therefore, it was not clear whether the measured decrease in specific binding in cultures from aged donors was due to a decreased number of activated cells or to a decreased number of binding sites per cell. In parallel to measurements of receptors, performed at different times following stimulation (20 and 44 h), numbers of cells in the different early cell cycle phases G0, G1a and G1b were quantitated by flow cytometry. In this way the receptor number per cell in each phase of the cell cycle could be determined. Receptor numbers on resting cells and in the earliest phase of activation (G1a) were found not to be influenced by age. A decreased receptor density, however, was apparent on G1b cells from aged donors.