Major surgery impairs cellular and humoral immunity, in particular natural cytotoxicity, and this may facilitate the dissemination of tumour cells in the peri-operative period. Recombinant interleukin-2 (rIL-2) has been used to modulate peri-operative immune function. Eighteen patients were randomized to receive either rIL-2 or placebo for three days, as a subcutaneous injection, prior to surgical resection for colorectal cancer. Natural cytotoxicity (natural killer (NK) and lymphokine-activated killer (LAK) cell activity), monocyte phagocytosis and immune cell surface activation marker (CD14+HLA-DR) expression were assessed during therapy and for up to 21 days after surgery. rIL-2 therapy enhanced both NK and LAK cell-mediated cytotoxicity and augmented circulating lymphocyte CD16+ and CD56+ cell subset populations. Circulating monocyte phagocytosis was also increased. Hence, rIL-2 may be used to enhance immune function in the peri-operative period in patients undergoing curative cancer surgery.