Abstract.A retrospective study revealed intestinal plasmacytosis in 53 of 102 rabbits used in various experimental studies and as controls. The breeds affected included New Zealand white (n = 46), Dutch-belted (n = 6), and Watanabe (n = 1) rabbits. Sex predisposition was not found in any breed. The mean (+ SD) ages were 3.1 I 1.4 years for New Zealand white rabbits, 1.3 +. 1.1 for Dutch-belted rabbits, and 2 years for the Watanabe rabbit. The severity increased with animal age. The incidence was higher ( P < 0.05) in rabbits used in antibody production and cholesterol studies. The lesions were characterized by multifocal to diffuse infiltration of well-differentiated plasma cells in the intestinal mucosa. Electron microscopic examination revealed typical plasma cell morphology of the infiltrating cells. Small intestine and cecum were the major sites affected. In severe cases, colon, rectum, trachea, esophagus, mesenteric lymph node, and spleen were also involved.Key words: Cecum; intestine; plasmacytosis; rabbits.The gut is continuously challenged immunologically with a myriad of dietary, microbial, and other environmental antigens and/or mi tog en^.^.^,^.^.^ As a part of the host mucosal immune system, gut-associated lymphoid tissues consist of inductive sites, e.g., Peyer's patches, appendix and solitary lymphoid nodules, and effector sites, e.g., the lamina propria.1.2,4.8 Under antigenic stimulation, naive T and B lymphocytes in the inductive sites undergo activation and differentiation into antigen-specific effector and memory T and B cells, including plasma cells. Activated T and B cells home to the effector sites via local and/or systemic circulation to mount a mucosal immune r e s~o n s e .~. * .~ Normally, plasma cells in the intestinal lamina propria constitute the largest plasma cell pool in the body, and most of these cells produce antigen-specific IgA.9 In this report, we describe plasma cell hyperplasia in the intestinal mucosa in 53 (52%) of 102 rabbits.Among the 102 rabbits studied retrospectively, 80 were New Zealand white (NZW), 14 were Dutch-belted (DB), and eight were Watanabe (WN) rabbits. The rabbits were supplied by eight commercial vendors. They were used or were planned to be used in various studies from 1988 to 1994 (Table 1). The rabbits were either found dead or euthanatized for evaluation of the concomitant diseases, e.g., anorexia, uterine carcinomas, mammary gland tumors, and pneumonia. Tissues were fixed in 10% neutral buffered formalin, embedded in paraffin, and stained with hematoxylin and eosin (HE). Selected tissue samples were fixed in Karnovsky's fixative for transmission electron microscopy. Histologic diagnosis of intestinal plasmacytosis was established by estimation: 1) if >50% of the infiltrating cells in the lamina propria were plasma cells and 2) if the plasma cells infiltrated >25% of the lamina propria area in depth and/or length on a given histologic section (Figs. 1-4). Data were analyzed with a two-tailed t-test (Statwork@, Cricket, Philadelphia PA) and x2 test.I...