The purpose of this study was to determine in healthy humans whether First Leaf (FL; composed of blackcurrant extract powder, lactoferrin and lutein) and Cassis Anthomix 30 (CAM30; blackcurrant extract powder) can positively modify the colonic microbiota by enhancing the growth of the beneficial bacteria and inactivating the toxic bacterial enzymes which are known to be involved in colonic carcinogenesis. Thirty healthy adult male and female volunteers were recruited for this study. Fluorescent in situ hybridization was carried out to analyse the populations of fecal microbiota. Consumption of FL and CAM30 led to significant increases (P < 0.0001) in the population sizes of lactobacilli and bifidobacteria whereas the population sizes of Clostridium spp. and Bacteroides spp were decreased significantly (P < 0.0001). In addition, feeding of FL and CAM30 decreases the activity of β-glucuronidase (bacterial enzyme which is considered to be one of the enzymes that increases risk for colorectal cancer) and significantly decreased (P < 0.05) the fecal pH. In conclusion, the results of this study open up the possibility that consumption of FL and CAM30 can offer various benefits to human health through acting as novel prebiotic agents via increasing the numbers of beneficial bacteria (lactobacilli and bifidobacteria) in the gut. Copyright © 2013 John Wiley & Sons, Ltd.