Three patients with renal adenocarcinoma and one with metastatic malignant melanoma were treated with continuous intravenous infusion of 18 x 10(6) IU/m2/day interleukin-2 during 5 days per week (2 weeks). Overall 60% of the calculated dose was administered owing to the development of severe toxicity. Among the hematological effects, eosinophilia was found in all patients, which was more marked 15-20 days after therapy was started. In addition, 15% of atypical lymphocytes were found in peripheral blood. These cells, denominated Pinocchio cells, show a cytoplasmatic prolongation with azurophilic granulation. Their cytochemical study disclosed a marked positivity for acid phosphatase and alpha-naphthyl acetate esterase, and a variable positivity for dipeptidyl-amino peptidase (DAP IV). The immunophenotype revealed that is a T lineage cell population, basically CD 3+, with small or absent positivity for the monoclonal CD 19 antibody. The presence of Pinocchio cells, which are effector cells mediating tumor destruction by an apoptosis mechanism, is related to the administered dose of interleukin-2.