Several groups of investigators, using varied tech niques, have recentiy demonstrated the existence of circulating antigen-antibody complexes in the inflam matory bowel diseases. This phenomenon may be an important clue that can tie together some of the diverse clinical and immunological findings in these conditions. I, 2 As an example, the deposition of these antigen-antibody complexes in other tissues could read ily explain some of the extragastrointestinal serum sickness manifestations of the inflammatory bowel diseases (IBD).3 Such localization of inflammatory reac tions are common in other diseases in which circulating complexes are found. 4 , 5 The deposition of these com plexes in blood vessel walls 3 , 6 may explain the vasculitis that is seen not infrequently in both ulcerative colitis 7 and Crohn's disease. s Even granulomata formation re sulting from the deposition of these complexes has been reported. 8 L)- mphocytes obtained from both ulcerative colitis and Crohn's disease patients have been shown to be cytotoxic for colonic cells in tissue culture. 9 - 11 Interest ingly, if normal lymphocytes are incubated in the sera obtained from inflammatory bowel disease patients, these lymphocytes will also exhibit this colonic cytotox icity; on the other hand, if the cytotoxic lymphocytes obtained from individuals with ulcerative colitis or Crohn's disease are incubated in heterologous IBD sera, these lymphocytes lose their cytotoxicity. 12, 13 Yet, a loss of cytotoxicity does not result if cytotoxic lymphocytes are incubated in autologous sera. 12 , 13 These above results could readily be explained by the presence of antigen-antibody complexes. For instance, Perl mann and Holm have shown that antibody or antibody complexed to target cells can induce lympho cyte cytotoxicity to the appropriate target antigen. 14 lED sera, therefore, could induce colon cytotoxicity in normal lymphocytes if those sera contain colon antibody or colon antibodies-antigen complexes. Earlier, it had been demonstrated that IBD sera did contain such an antibody that would react with fetal colon 15 as well as with an antigen which had similar antigenic determi nants and is present in many intestinal bacteria. 16. 17 Considerable quantities of this latter, highly immuno genic, 18 common heterogenetic antigen must be availa ble in the intestinal lumen; furthermore, its absorption into the circulation has been demonstrated in the inflammatory bowel diseases_ 19, 20 The presence, then, of colon antibodies and/or antibodies to the cross-reacting bacterial antigens as well as the bacterial antigen itself