Little is known about organ associated tumor response to systemic interleukin 2 (IL2) therapy. The effect of IL2 on bladder cancer growth in the skin and in the genitourinary tract was investigated. C3H mice were implanted with the syngeneic transitional cell carcinoma, MBT-2, intradermally (i.d.), beneath the left renal subcapsular area, and in one experiment, simultaneously in the bladder. IL2 (human recombinant form; Biogen Research Co) was given i.p. at 5000 U thrice daily for 5 consecutive days commencing on Day 3, or for 10 to 11 days commencing on Day 10 with some doses omitted at signs of toxicity. For comparison, mice bearing 3-d and 10-d tumors in the skin and subcapsular kidney were treated with chemotherapy (cisplatin, 6 mg./kg. X 3; mitomycin C, 3 mg./kg. X 3; cyclophosphamide, 75 mg./kg. X 1). IL2 therapy mediated growth suppression of 10-d tumors in the genitourinary organs and skin at a similar rate. In contrast to IL2, systemic chemotherapy mediated tumor suppression in an organ specific manner; renal subcapsular tumors responded to the chemotherapy, whereas i.d. tumors were insensitive. Three-day tumors (both i.d. and renal subcapsular tumor) responded relatively well to each treatment compared to 10-d tumors. These data suggest that in systemic immunotherapy with IL2, anatomic location of the tumor is less important for inducing an antitumor response than in chemotherapy.