It is well known that in the elderly a deterioration of immune functions may occur. Particularly, stimulation of T cells from aged individuals leads to different kind and/or size of responses if compared with the responses obtained from T cells from young individuals. At the same time, an increase in prevalence of autoantibodies occurs in elderly. The altered production of certain cytokines might explain this paradox of decreased responsiveness to foreign antigens in the face of an increased response to self-antigens. We and others have suggested that this kind of immune response might depend on an age-associated impairment of Th-1 type function that selectively affects production of cytokines involved in the control of cellular responses. In contrast, Th-2 type function is seemingly not affected in elderly, as suggested by normal in vitro production of cytokines involved in humoral responses. To strengthen this hypothesis, in this study we have analysed the influence of age on the ability of mitogen-stimulated cultures of peripheral blood mononuclear cells from human beings to produce another Th-2 type cytokine, i.e. interleukin-5 (IL-5). IL-5 content of both 24- and 48-h stimulated cultures from old individuals was greater than that of young ones, although this difference attained significance only at 48 h. We suggest that the decreased production of Th-1 type cytokines in the presence of a normal or even increased production of Th-2 type cytokines might account for the pattern of immune response which may be observed in elderly, i.e. a normal or increased humoral response, including an autoimmune one, in the face of a low cell mediated immune response.