Several immune parameters--particularly T-cell dependent immune responses--are altered in aged subjects. To test the hypothesis that they may be the consequence of more general age-related lymphocyte biochemical alterations, and particularly of the energy producing system, the effect of L-carnitine and acetyl-L-carnitine on cell proliferation was studied in peripheral blood lymphocytes from donors of different ages. The results showed that phytohaemagglutinin-induced peripheral blood lymphocyte proliferation was markedly increased in L-carnitine- or acetyl-L-carnitine-preloaded lymphocytes from young and especially from old subjects. Cells from aged subjects considerably improved their defective proliferative capability. Preliminary observations suggest that L-carnitine-preloading also protected peripheral blood lymphocytes from old donors when such cells were exposed to an oxidative stress.