In order to determine molecules involved in the differentiation and proliferation of human CD8+ cells, two ex vivo expansion models were established: coculture of freshly purified human CD8+ cells with irradiated autologous feeders (AF) or stimulation with anti-CD3. Two different proliferation kinetics of CD8+ cells and expression patterns of CD57 were observed between these conditions. Differential display reverse transcriptase–polymerase chain reaction was applied to investigate the differential expression of mRNA species between CD8+ CD57+ and CD8+ CD57− populations. A differentially expressed RNA species called alpha nascent polypeptide associated complex (α NAC) was found at a higher level in CD8+ CD57− cells than in CD8+ CD57+ cells. In the presence of AF, the expression of α NAC was reduced on culturing whilst proliferation increased. Similarly, in cultures stimulated with anti-CD3, α NAC reverted to its inactive form and differentiation and proliferation increased. Using a phosphorothioate-modified oligodeoxynucleotide antisense directed specifically against α NAC mRNA, protein expression was inhibited and increased CD8+ cell proliferation and CD25 expression were observed irrespective of the culture conditions. This suggests that α NAC protein is antiproliferative molecule. This is the first description of the function of the α NAC protein in human CD8+ T cells.