The chemical properties, pharmacology, immunology, pharmacokinetics, clinical trials, adverse effects, and dosage and administration of recombinant interleukin-2 are reviewed. Recombinant interleukin-2 is an immunomodulating agent that stimulates the proliferation, activation, and differentiation of T and B cells, natural killer cells, and thymocytes. Two recombinant interleukin-2 products, aldesleukin and teceleukin, have been extensively studied. Most clinical experience with recombinant interleukin-2 has involved the treatment of renal cell carcinoma, melanoma, and colorectal cancer with a National Cancer Institute protocol. Patients with renal cell cancer and melanoma, who historically respond poorly to conventional therapy, have responded to therapy with recombinant interleukin-2. Recombinant interleukin-2 has been administered alone and in combination with lymphokine-activated killer cells, tumor-infiltrating lymphocytes, and interferons alfa and beta. In addition, the effect of dosage, administration rate, dosage schedule, route of administration, and cyclophosphamide pretreatment have been investigated. The adverse effects of recombinant interleukin-2 are generally reversible but are frequently severe and dose-related. Dose-limiting adverse effects include hypotension, edema, and renal dysfunction. Since hemodynamic monitoring and supportive care are essential, recombinant interleukin-2 should be administered in a critical-care setting by trained personnel. Recombinant interleukin-2 represents an advance in the therapy of renal cell cancer and melanoma and offers a new approach to the treatment of other refractory or recurrent malignancies.