A colony immunoblotting method has been developed to allow detection of the probiotic Bifidobacterium animalis strain DN-173 010 in human faecal samples. Rabbits were immunized with heat-killed DN-173 010 bacteria resulting in the production of an antiserum highly specific for bacteria belonging to Bif. animalis species. Of the 89 strains representative of 29 different bifidobacterial species tested, only the 15 strains of the Bif. animalis species could be detected with the antiserum. In Western immunoblotting the serum reacts with a protein of 45-kDa apparent molecular weight. None of the bacteria classically encountered in human faecal samples and able to grow on non-selective Columbia blood agar (enterobacteria, Bacteroides or Lactobacillus for instance) reacted with the antiserum. Taking advantage of the high specificity of the antiserum and of the absence of Bif. animalis bacteria in faeces samples of five human volunteers, we demonstrated that strain DN-173 010 survives the intestinal transit. Being based on a combination of semiselective cultivation and colony immunoblotting techniques, the method allowed detection of the Bif. animalis strain even when it represented only one thousandth of the total bifidobacterial population.