Peripheral blood mononuclear cells (PBMC) were taken by leukapheresis from a patient with melanoma skin metastases and stimulated in vitro using 1000 IU recombinant interleukin 2 (IL-2)/ml to generate lymphokine-activated killer cells (LAK cells). Two-colour immunofluorescence analysis demonstrated an IL-2-induced up-regulation of CD25 on natural killer cells (CD56+) as well as on T lymphocytes (CD3+). After radiolabelling with indium-111, the cells were reinfused. Gamma-camera imaging revealed an enrichment at the tumour sites. Immunostaining of tumour tissue taken before and after scintigraphy demonstrated CD25+ T lymphocytes (CD2+, CD3+), but no natural killer cells (CD16+, CD56+) infiltrating the metastases. LAK cell enrichment at melanoma metastases in vivo did not involve natural killer cells, but was characterized by increased numbers of activated T lymphocytes in this patient.