The changes in transmembrane electrical potential (TMP) of Concanavalin A (Con-A)-stimulated lymphocytes from young adult and aged CBA/Ca mice were studied with a potential-sensitive fluorescent oxonol probe. The initial effect of Con-A was to depolarise lymphocytes from young mice and abrogated in the presence of tetraethylammonium chloride (TEA), an inhibitor of K(+)-selective channels. Young and old T lymphocytes both responded to the calcium ionophore A23187 by becoming hyperpolarized, but this occurred more slowly in the old cells. While treated with the ionophore, old B cells appeared to be limited in their ability to depolarize in the presence of high external K+ concentrations, which did not hold for T cells of old animals. One or more defects in the mechanisms of monovalent ion transport across the membrane of old lymphocytes are probably responsible for these differences and may be associated with the known age-related dysfunction of the immune system.