HLA phenotypes were characterized for 79 patients with metastatic renal cell carcinoma treated with interleukin 2 (IL-2). HLA-A32 was associated with a clinical response (P = 0.025). The frequency of HLA-A3 and/or A32 was higher among responders than non-responders (P = 0.008). Thus, these results suggest that, in vivo, IL-2 may enhance cellular-mediated immunity against a tumour antigen and that some MHC molecules are more efficient than others for endogenous tumour antigen presentation.