Dietary supplementation with glutamine (Gln), arginine (Arg) or ornithine 2-oxoglutarate (alpha-ketoglutarate; OKG) has attracted recent attention for the potential to improve anti-cancer immune function. However, since these compounds have not been compared systematically in an internally controlled study, their relative efficacy is difficult to estimate. Buffalo rats were fed on nutritionally complete semi-purified diets supplemented with Gln, Arg or OKG for 14 days after implantation of the Morris hepatoma 7777 (n>/=7 per diet). The control diet was made isonitrogenous and isoenergetic by addition of a mixture of non-essential amino acids. After 14 days, peritoneal macrophages and splenocytes were isolated to determine cell phenotypes, macrophage cytostatic activity and natural killer (NK) cell cytotoxicity, as well as nitric oxide (NO) and cytokine production. Diet had no effect on tumour weight (1.6+/-0.2 g; n=59). However, rats fed OKG had increased macrophage cytostatic activity and NK cell cytotoxicity (P<0.05). Although enhanced killing ability by NK cells was associated with higher splenocyte NO production (P<0.04), increased cytotoxicity was not inhibited by a specific inhibitor of inducible NO synthase. The proportion of interleukin-2-receptor-positive T cells after stimulation increased in rats fed OKG (P<0.05); however, cytokine production was not affected by diet. None of OKG, Gln or Arg altered tumour growth compared with a control mixture of non-essential amino acids. These results suggest no net advantage for anti-cancer immunity, but do not preclude benefits in immune responses to disease recurrence or metastasis, therapy or secondary infection.