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SUMMARY To achieve optimum staining and reproducible counts of plasma cells in paraffin embedded tissue with the immunoperoxidase technique we have found it essential to obtain a plateau count by titration of antisera for each specimen. This modification was used to study IgA, IgM, IgE, and IgG plasma cells in rectal biopsies from 20 controls, 20 patients with ulcerative proctocolitis, 20 with Crohn's colitis, 20 with non-specific proctitis, 15 with bacterial colitis, and seven with Crohn's disease but no apparent large bowel involvement. Counts were correlated with the characteristic histological features of inflammatory bowel disease. In controls the ratio of the mean counts for IgA, IgM, IgE, and IgG plasma cells was 8:3:3:1. All types of plasma cells were very significantly increased in the patients with ulcerative proctocolitis, Crohn's colitis, and non-specific proctitis and counts correlated with the severity of inflammation. There was no significant difference between the counts in these three groups. All counts tended to be higher in bacterial colitis than in controls, the difference being significant for IgA and IgE. When matched for severity of inflammation there was no significant difference between the counts in bacterial colitis and inflammatory bowel disease. The counts in patients with Crohn's disease but no large bowel involvement were not significantly different from controls. These results suggest that changes in plasma cell counts in inflammatory bowel disease are a non-specific reponse to mucosal damage, possibly by a luminal irritant, and do not differentiate the type of inflammatory bowel disease.Ulcerative proctocolitis and Crohn's colitis are common diseases of unknown aetiology in which imunological mechanisms play a part. The differentiation of these two conditions, although often very difficult, is of practical importance in relation to both medical and surgical treatment. Non-specific proctitis is usually considered a mild variant of ulcerative proctocolitis but there are a number of differences,l which suggest they might be separate conditions. Changes in the total and differential rectal mucosal plasma cell counts have been reported in all three conditions.2~9 The different studies have given strikingly conflicting results, however, and there is even great variation between the results in control groups.3 %8 1012 There are probably several reasons for the conflicting results. One is the small numbers of patients studied -for controls the average number is nine, for ulcerative