Interleukin-2 (IL2) binds with high affinity to the IL2 receptors overexpressed on activated T lymphocytes in various pathologic conditions. Radiolabeling of IL2 with a positron-emitting isotope could provide a tool for noninvasive PET of activated T cells in immune-mediated diseases. We report the labeling of IL2 with N-succinimidyl 4-F-18-fluorolbenzoate (F-18-SFB) for the synthesis of N-(4-F-18-fluorobenzoyl)interleukin-2 (F-18-FB-IL2) and the in vitro and in vivo evaluation of this novel radiopharmaceutical for the detection of IL2 receptor-positive cells by PET. Methods: F-18-SFB was efficiently prepared using a 3-step radiochemical pathway. Purified F-18-SFB was incubated with IL2 in borate buffer (pH 8.5) and ethanol at 50 degrees C for 10 min. F-18-FB-IL2 was purified by reversed-phase high-performance liquid chromatography. As in vitro quality controls, a biologic binding assay, sodium dodecyl sulfate polyacrylamide gel electrophoresis, mass spectrometry, and 3-chloroacetic acid precipitation stability tests were performed. Biodistribution studies were performed in BALB/c mice to evaluate the distribution of the tracer in healthy animals. PET experiments were performed in severe combined immunodeficiency disease mice inoculated with phytohemoagglutinin-activated human peripheral blood mononuclear cells (hPBMc). Whole-body images were acquired 30 min after injection of 5-15 MBq of F-18-FB-IL2. Results: F-18-SFB was produced with a 34%-38% radiochemical yield. The radiochemical purity after solid-phase extraction purification ranged from 93% to 96%. Conjugation of F-18-SFB to IL2 yielded F-18-FB-IL2 as the major product. The radiochemical yield of F-18-FB-IL2 after high-performance liquid chromatography purification was 25%-35% based on F-18-SFB. F-18-FB-IL2 was stable in plasma at 37 degrees C and capable of stimulating T cells to an extent similar to native IL2. A biodistribution study showed highest tracer uptake in the kidneys and bladder due to rapid renal clearance of the tracer. Small-animal PET images showed binding of F-18-FB-IL2 to activated hPBMc proportional to the number of injected cells. Conclusion: We report the successful labeling of IL2 with F-18 for PET of activated T lymphocytes. F-18-FB-IL2 is stable, is biologically active, and allows in vivo detection of activated T lymphocytes.