This study evaluated the effect of a probiotic bacteria 'Bifidobacterium lactis', the carbohydrate 'resistant starch' (RS) and their combination (synbiotic), on their ability to protect against colorectal cancer (CRC). Bifidobacterium lactis has been shown previously to utilize RS as a substrate and up-regulate the acute apoptotic response to a carcinogen in the colon [Le Leu et al. (2005) J. Nutr., 135, 996-1001]. Sprague-Dawley rats were divided into six equal groups and fed semi-purified diets for 30 weeks. Colonic neoplasms were induced by 2 weekly injections of azoxymethane (15 mg/kg body wt). The experimental groups were as follows: control-no added dietary fibre or RS; RS in two forms-Hi-maize 958 or Hi-maize 260; B.lactis (lyophilized)-added to control and RS diets (six treatment groups in all). Rats fed RS in combination with B.lactis showed significantly lowered incidence and multiplicity of colonic neoplasms (P < 0.01) by >50% compared with the control group. There was a trend for protection by RS alone (P = 0.07), whereas no protection against cancer was seen in the group supplemented with only B.lactis. Fermentation events [short-chain fatty acid (SCFA), pH] were altered by the inclusion of RS into the diet, whereas the inclusion of B.lactis into the diet had no significant effect on the fermentation parameters. The synbiotic combination of RS and B.lactis significantly protects against the development of CRC in the rat-azoxymethane model. Synbiotic combination of prebiotic and probiotic seems likely to be a superior preventive strategy to prebiotic alone.