Non-infectious diseases such as CHD and certain cancers have become major causes of death and disability in affluent countries. Probiotics (principally lactic acid bacteria; LAB) may assist in lowering the risk of these diseases. Experimental studies with probiotics have given generally inconclusive outcomes for infectious disease and for biomarkers for non-infectious disease. In part this situation may reflect their inability to colonise the adult human gut effectively. Prebiotics can assist in promoting colonisation, and resistant starch (RS), as a high-amylose starch, is a prebiotic and synbiotic. This starch exerts its synbiotic action through adhesion of the bacteria to the granule surface. Consumption of RS assists in recovery from infectious diarrhoea in man and animals. A rice porridge, high in RS, appears to modify the autochthonous porcine large-bowel microflora favourably through lowering Escherichia coli and coliform numbers. Many of the beneficial effects of RS on large-bowel function appear to be exerted through short-chain fatty acids (SCFA)formed by bacterial fermentation. In man LAB are found in relatively highest numbers in milk-fed infants where the profile of fermentation products differs quite markedly from that in adults. It appears unlikely that ingestion of current probiotics will alter either total SCFA or the proportions of the major acids. More emphasis needs to be given to the investigation of the effects of complex carbohydrates, including RS, on the autochthonous microflora of the human large bowel.