OBJECTIVE Repeated intraperitoneal injections of anti-mouse pancreas rabbit serum or of anti-mouse pancreas guinea pig serum produce a chronical sclerotizing pancreatitis. This study has the aim to contribute to the further elucidation of the changes which occur in the acinar cells, as well as to the etiology and pathogenesis of immune pancreatitis, by means of immunohistological, enzyme histochemical and electron microscopic studies. METHODS Anti-mouse pancreas rabbit serum was obtained by sensitization of rabbits with an admixture of AB-mouse pancreas extract (100,000 g - supernatant) and complete Freund's adjuvant [details see NIZZE, Exp. Path. (1975a)]. The presence of precipitating mouse pancreas antibodies in the rabbit serum was ascertained by the agargel diffusion test according to Duchterlony (1958). The experiments were performed with 54 adult male white mice (AB colony strain) of 22 to 30 g.b.s. (averagely 26 g). The animals were divided into 4 groups which were treated as follows: 1. 24 mice with anti-mouse pancreas rabbit serum, 2. 12 mice with rabbit normal serum, 3. 12 mice with physiological saline, 4. 6 mice remained untreated (controls) Always 4 animals of the group 1 as well as each 2 of the groups 2 and 3 were administered in total 1, 3, 5, 9, 17 or 33 intraperitoneal injections of 0.3 ml of the correspondent serum or with physiological saline within 3 hours, 1, 2, 4, 8 or 16 days. The last injection was regularly applied 3 hours before sacrification by decapitation. The time of sacrification was always at 11.00 o'clock a.m. For immunohistological and enzyme histochemical investigations 10 mum thick cryostat sections were prepared consisting of pancreatic specimens piled up to a bloc. In each case the tissue samples were taken from the experimental animals and from one control animal sacrificed at the same day. The sections were incubated in FITC-labelled anti-rabbit globulin goat serum at room temperature for 30 min in a moist chamber. For control of specificity were employed: a) initial incubation of equal sections with unlabelled anti-rabbit globulin goat serum for 30 min (""blocking test''), b) pancreatic tissue specimens of each one untreated control animal present in the cryostat sections and thus incubated like the pancreatic tissue of the experimental animals, c) native nonincubated cryostat sections from the same bloc to exclude nonspecific autofluorescence. Evaluation of the sections was done in a Zeiss-Lg-microscope with HBO-50 high pressure mercury lamp. Exciter filters were UG 1/3.5 and 1/1.5, the eyepiece was screened with a GG 9/1 filter photographs were taken on ORWO X-ray film RS 2 (VEB Filmfabrik Wolfen). The enzyme histochemical studies were performed on cryostat sections of the same tissue bloc using the following methods: lead nitrate- or calcium-Co-method after GOMORI (1952) for demonstration of acid and alkaline phosphatase, naphthylacetate method (NACHLAS and SELIGMAN 1945) for nonspecific esterase, MTT-co-method (PEARSE et al...