Forty two cases (46 lesions) of inflammatory fibroid polyp (IFP) of the stomach were reviewed histologically and studied immunohistochemically. The paraffin sections were stained with a panel of antibodies against alpha-smooth-muscle actin, HHF-35, desmin, vimentin, lysozyme, alpha-1-antitrypsin, alpha-1-antichymotrypsin, KP1, Mac 387, S-100 protein, neuron-specific enolase, factor VIII RAg, and with Ulex europaeus agglutinin I. The lesions ranged in size from 0.3 to 3.5 cm with a mean of 1.2 cm. Forty (87.0%) IFPs occurred in the antrum, five in the gastric body, and one in the cardia. In all but two lesions, the mucosal layer was involved, and 20 (43.5%) lesions were entirely restricted to the mucosa. All 46 IFPs tested diffusely positive with vimentin. Positive reactions for alpha-smooth-muscle actin and HHF-35 were observed in 12 (26.1%) and 10 (21.7%) IFPs, respectively. Two lesions with alpha-smooth-muscle actin and one lesion with HHF-35 showed a diffuse staining. Seventeen (37.0%) lesions were focally positive for KP1, and seven (15.2%) IFPs were focally positive for Mac 387. All other antibodies and agglutinin were negative with the proliferating cells. The results of this study confirmed (a) the presence of myofibroblastic and histiocytic lines of differentiation in addition to the main fibroblastic features for the main cellular component in IFP, and (b) the heterogeneity of the immunoprofile of IFP.