The immune response in a patient to an infection (a perirectal abscess) with Bacteroides fragilis ss. fragilis and Clostridium difficile was investigated with the use of agglutination tests, the Ouchterlony technique and indirect immunofluorescence (IFL) technique. Antibodies against B. fragilis ss. fragilis were demonstrated with all the three methods while antibodies against C. difficile were recorded only with indirect IFL technique. A significant decrease of the antibody titres (fourfold or more) as measured with agglutination tests and indirect IFL tests and a disappearance of precipitating antibodies, as demonstrated with the Ouchterlony technique, were observed during a five month period. Serum specimens from blood donors, serving as controls, showed no reactivity with the antigens used. The results also showed antigenic differences between strains of B. fragilis ss. fragilis, indicating the possibility of more than one serotype within this subspecies of B. fragilis.