Twenty-two subjects (11 HLA A1 B8 DR3, 11 non-A1 B8 DR3) were tested for the capacity of their lymphocytes to express Tac molecules and interleukin-2 (IL-2) receptors (quantified using radiolabelled IL-2) after mitogen stimulation. Ten of these subjects (five A1 B8 DR3 and five non-A1 B8 DR3) were also tested for the ability of their lymphocytes to proliferate under IL-2 stimulation. A1 B8 DR3 subjects express a normal number of high-affinity IL-2 receptor sites, but the affinity of these receptors sites is significantly increased. Unexpectedly, A1 B8 DR3 lymphoblasts show a lower response to IL-2 than non-A1 B8 DR3 for high doses of recombinant IL-2.