CD4+ cells from young (3 months) and old (19 months) mice were stimulated by plate-bound anti-CD3 monoclonal antibody (mAb) alone or also by soluble anti-CD28 mAb. Supernatants were analysed by enzyme-linked immunosorbent assay (ELISA) to determine cytokine concentrations. Total RNA was extracted from cells, reverse transcribed and the cDNA amplified by polymerase chain reaction (PCR) to evaluate the amount of specific mRNA. The results indicate that anti-CD3 alone is not sufficient to induce interleukin-2 (IL-2) production in CD4+ cells from both young and old mice. However, anti-CD28, together with anti-CD3 mAb, induces a much higher production of IL-2 in CD4+ cells from young as compared with old mice. Conversely, interferon-gamma (IFN-gamma) production is also induced by anti-CD3 alone and is higher in CD4+ cells from old as compared with young mice. Upon addition of anti-CD28 mAb, IFN-gamma production increases in both groups, but it remains much higher in old than in young mice. Also the production of IL-4 and IL-10 is induced by anti-CD3 mAb but it is increased by the addition of anti-CD28 mAb. CD4+ cells from old mice produce more IL-4 and IL-10 as compared with cells from young mice. The amounts of cytokine specific mRNA in CD4+ cells from young and old mice parallel the cytokine levels in culture supernatants. Results on the mRNA turnover indicate that when CD4+ cells are stimulated by anti-CD3 or costimulated also by anti-CD28 mAb, the IFN-gamma, IL-4 and IL-10 specific mRNAs are more stable in old than in young mice, suggesting that mRNA stability has a relevant role in the different patterns of cytokine production.