CD23 is considered as an activation marker. Its expression on lymphocytes was studied during aging by using 1- to 142-week-old rats. Before analysis, all animals were injected with monoclonal IgE, in order to detect their full expression of CD23 molecules on their lymphocyte surface membranes. In elderly rats, a decrease was observed in the percentages of CD23+ lymphocytes from mesenteric lymph nodes, spleen, Peyer's patches and peripheral blood. In addition, these lymphocytes expressed significantly fewer CD23 molecules per cell. Almost all CD23+ cells belonged to the B lineage and were bearing both IgM and IgD on their membranes. It is suggested that a decrease in the number of both IgM+IgD+CD23+ lymphocytes and CD23 molecules per cell may contribute to immune deficiency in the elderly.