Stool specimens from 17 multiple sclerosis (MS) patients were examined for bifidobacteria. The count of colony forming units (CFU)/g faeces and the differentiation and typing of species, especially of Bifidobacterium adolescentis biotypes were performed. The pH of the samples was also measured. Stool specimens from 17 healthy volunteers were examined as a control by the same methods (a modified medium, suitable for the detection and recognition of bifidobacteria in adults is given). The results demonstrate a significant deficiency in bifidobacteria of the adolescentis group in MS patients. In 3 patients, no bifidobacteria could be detected (less than 10(7]. In comparison with the samples from healthy donors, B. adolescentis "c" was reduced and B. adolescentis "a" could not be detected at all in MS patients' stool specimens. The occurrence of B. adolescentis "b" and B. adolescentis "d" as well as the species B. bifidum and B. longum did not significantly differ within both groups. The pH was only slightly increased in the MS group.