General surgical procedures are followed by a period of generalized immunosuppression that may favour the deposition of metastases seeded at operation in patients with malignant disease. In an attempt to prevent the suppression of host-antitumour immune mechanisms following surgery we have studied the immunological effects of low-dose perioperative interferon-alpha (r-HuIFN alpha). Patients were randomly allocated pre-operatively to the control (n = 15) or treatment group (n = 15). Patients in the treatment arm received a 1-week course of subcutaneous recombinant human interferon-alpha 2a (Roferon-A) at a dose of 2 megaunits daily starting on the evening before surgery. Natural killer cell, lymphokine activated killer cell cytotoxicities and endogenous interleukin 2 production were measured 1 day before surgery and on the first, third, fifth and tenth postoperative days. Treatment with r-HuIFN alpha did not prevent the postoperative impairment of interleukin 2 production or lymphokine activated killer cell cytotoxicity. However it prevented the fall in natural killer cell activity normally observed following surgery. This may have important consequences in controlling metastatic dissemination of tumour in this vulnerable period.