The effects of total parenteral nutrition (TPN), using a lipid-based regimen, on non-specific lymphocyte function and tumour-directed cellular cytotoxicity was studied in 30 patients suffering from gastrointestinal (GI) cancer. After 7 days of TPN, augmented lymphocyte blastogenesis and production of the helper T-lymphocyte lymphokine Interleukin-2 were observed. However, over the same time period, significant impairment of basal natural killer (NK), and IL-2 activated NK activity were observed. Furthermore, lymphokine-activated killer (LAK) cell responses towards the NK resistant cell line DAUDI and the colorectal tumour cell line COLO 320, were markedly depressed. These findings have important implications for the use of this TPN regimen in GI cancer patients who might be considered for either surgical adjuvant or primary treatment with immunotherapy protocols.