Interleukin-2 (IL-2) therapy has resulted in modest response rates in patients with renal carcinoma and melanoma, but few reproducible pre- or post-treatment parameters have been associated with response, and tumour localization with lymphokine-activated killer cells has only occasionally been demonstrated. Eight patients (seven with renal carcinoma and one with melanoma) treated on a protocol with chronic indomethacin and ranitidine with three courses of continuous infusion of IL-2 had peripheral blood leukocytes withdrawn 12–36 h after completion of IL-2 therapy, labelled with 111In tropolone and reinjected. Images were taken at 4 and 20 h after reinjection. Three of these patients achieved objective responses to therapy, but none demonstrated uptake of radioisotope-labelled leukocytes in known tumour-bearing areas. Two nonresponding patients (one renal carcinoma, one melanoma) demonstrated uptake in all known tumour areas; one further nonresponding patient demonstrated uptake in the region of a femoral metastasis, but not in other bulky areas of disease. No correlation between scan uptake and leukocyte subsets could be demonstrated. Although occasional patients demonstrate tumoural accumulations of 111In-labelled leukocytes after completion of therapy with IL-2, this does not appear to be associated with response.