Anaerobes, especially the gram-negative non-spore-forming bacilli of the indigenous biota, are recognized as important agents of clinical infection; however, information regarding human antibodies to anaerobes is limited. Sporadic work, employing agglutination, gel diffusion, passive hemagglutination, and immunofluorescence, demonstrated antibody response to various Bacteroidaceae in patients with infections caused by Bacteroides or Fusobacterium. Because of the need for using homologous (autologous) isolates and the apparent antigenic heterogeneity of strains, there has been no general clinical application of these findings. Increased concentrations of antibodies to anaerobic intestinal organisms in chronic inflammatory bowel diseases have been found; however, it is unclear whether these are the effect or cause of these conditions. Natural antibodies to Bacteroidaceae, primarily of the IgM class, are widely distributed in normal humans; thus for serologic diagnosis IgG antibodies should be sought. Radioimmunoassays and various immunoelectorphoretic methods for detection of both antibody and antigen are presently being evaluated for diagnostic use in infections due to anaerobic bacteria.