Prebiotics, probiotics and synbiotics are dietary ingredients with the potential to influence health and mucosal and systemic immune function by altering the composition of the gut microbiota. In the present study, a candidate prebiotic (xylo-oligosaccharide, XOS, 8 g/d), probiotic (Bifidobacterium animalis subsp. lactis Bi-07, 10(9) colony-forming units (CFU)/d) or synbiotic (8 g XOS + 10(9) CFU Bi-07/d) was given to healthy adults (25-65 years) for 21 d. The aim was to identify the effect of the supplements on bowel habits, self-reported mood, composition of the gut microbiota, blood lipid concentrations and immune function. XOS supplementation increased mean bowel movements per d (P=0.009), but did not alter the symptoms of bloating, abdominal pain or flatulence or the incidence of any reported adverse events compared with maltodextrin supplementation. XOS supplementation significantly increased participant-reported vitality (P=0.003) and happiness (P=0.034). Lowest reported use of analgesics was observed during the XOS + Bi-07 supplementation period (P=0.004). XOS supplementation significantly increased faecal bifidobacterial counts (P=0.008) and fasting plasma HDL concentrations (P=0.005). Bi-07 supplementation significantly increased faecal B. lactis content (P=0.007), lowered lipopolysaccharide-stimulated IL-4 secretion in whole-blood cultures (P=0.035) and salivary IgA content (P=0.040) and increased IL-6 secretion (P=0.009). XOS supplementation resulted in lower expression of CD 16/56 on natural killer T cells (P=0.027) and lower IL-10 secretion (P=0.049), while XOS and Bi-07 supplementation reduced the expression of CD19 on B cells (XOS x Bi-07, P=0.009). The present study demonstrates that XOS induce bifidogenesis, improve aspects of the plasma lipid profile and modulate the markers of immune function in healthy adults. The provision of XOS + Bi-07 as a synbiotic may confer further benefits due to the discrete effects of Bi-07 on the gut microbiota and markers of immune function.