Peripheral blood lymphocytes (PBL) from old subjects were less responsive to PHA than those from young controls, in both groups the PBL content of macrophages, T and B cells were comparable. Macrophages were identified as peroxydase containing cells, rosette formation with sheep red blood cell was used as a T cell marker and immunoglobulin bearing lymphocytes were considered to be B cells. PBL were separated in two subpopulations of purified T cells (F1 and F2) differing in their responsiveness to PHA and in their ability to cooperate with B cells. Purified B cells had a synergistic effect on the F1 response to PHA whereas they were often suppressive for the corresponding F2 response. Peroxydase containing cells enhanced the response of unfractionated total PBL and of purified T cells in the young subjects. The depressed response of the old age group could be related mainly to a suppressive effect of monocytes and a decreased circulating pool of F1 T cells.