In the course of a phase I trial, in which recombinant IL-2 (rlL-2) was infused intraperitoneally (i.p.) in patients with peritoneal carcinomatosis, we evaluated the effect on "tumorassociated lymphocytes" (TAL) isolated from the axitic fluid. No major changes in the percentages of cells expressing the CD3, CD4, CD8, Leu-7, OKMl and WT-31 antigens were detected either in TAL or in peripheral blood lymphocytes (PBL) after 7 days of rlL-2 infusion. In contrast the percentages of TAL (but not PBL) expressing surface IL-2 receptor (Tac), or LAK-I antigen were sharply increased. Analysis of cytolytic functions showed a potentiation of the lytic activity against natural-killer (NK) sensitive K562 target cells and the de novo appearance of lytic activity against fresh melanoma cells. In one patient IFN--y was detected in the ascitic fluid following rlL-2 infusion. T-cell clones derived from the patient were analyzed for the IFN-y production. While only approximately 40% of PB-derived control clones produced medium to low amounts of IFN-T, all of the TAL-derived clones produced medium to high amounts of the lymphokine.