Summary:Patients with metastatic solid tumors were treated with six escalating doses of weekly intralymphatically injected recombinant interleukin 2 (i.l. IL‐2). Nine patients completed the treatment and were evaluated for immunologic features of their peripheral blood lymphocytes (PBLs). The patients' PBL counts increased 4 days after the first i.l. IL‐2 injection. The cell counts remained higher than baseline in week 6 prior to the last i.l. IL‐2 injection. However, the PBL number decreased below baseline 1 day after the sixth injection, and recovered to normal levels after 3 days more. Natural killer (NK) activity showed similar changes when calculated as total activity per ml of blood. In vitro 1 h treatment of PBLs with IL‐2 greatly enhanced NK cyto‐toxicity. The enhancement was only slight in the first week of i.l. IL‐2 treatment, but was significantly greater on day 35 (7 days after dose 5) and day 39 (4 days after dose 6). In contrast, the increase was similar to the baseline on day 36, the day after the sixth injection. No lymphokine‐activated killer activity was detected in the patients' PBLs with or without short‐term in vitro IL‐2 treatment. Besides the NK cytotoxic function, lymphoid subpopulations were evaluated numerically for total T cells (CD3/OKT3), T‐cell subsets (CD4/OKT4 and CD8/OKT8), B cells (OKB7), NK cells (CD56/NKH1/Leu19, CD16/Leu11), and monocytes/NK cells (CD11b/OKM1). The activation markers (HLA‐DR, CD25/Tac, and CD38/OKT10/Leu17) were also included. Intralymphatic IL‐2 treatment had no effect on the PBL surface marker expression in the first week of treatment. However, by week 6, the percentages of cell populations expressing the NK‐associated antigens CD56, CD16, and CD11b were significantly increased. In contrast, the percentage of CD3‐positive T cells showed no change or a marginal decrease. Prior to and after i.l. IL‐2 treatment, the CD56‐positive cells in the PBLs were predominantly CD16 positive and CD3 negative. The i.l. IL‐2 treatment did not induce PBL proliferation, or changes in the expression of CD25 (Tac), HLA‐DR, CD38, CD4, CD8, CD57, or OKB7 in the patients' PBL. These results indicate that i.l. IL‐2 treatment does affect the total number of PBLs, the cells expressing NK activity, and NK‐associated surface markers.