The exact cell type and site(s) involved in interleukin‐1 (lL‐1) production during gingival inflammation was determined by combining immunohistochemistry andin situhybridization. IL‐1 messenger RNA (mRNA)‐expressing cells in human inflamed gingiva were identified as macrophages. The rate of IL‐α mRNA expression in these macrophages was the same as IL‐1βmRNA expression. The rate of IL‐1 mRNA expression was higher in connective tissue furthest from the pocket epithelium, although more macrophages were present at the connective tissue subjacent to the pocket epithelium. The IL‐1 activity in gingival crevicular fluid (GCF) obtained from inflamed gingiva was higher than that from healthy gingiva and decreased after periodontal therapy. The IL‐1 activity in GCF was almost completely abolished by the addition of anti‐IL‐1α antibody but not by anti‐IL‐1βantibody, indicating that IL‐1α is the predominant form in GCF. However, the IL‐1 activity in GCF was unrelated to the number of IL‐1 mRNA‐exprerssing macrophages in the same gingival site where the GCF was obtained at the same time. The results suggest that macrophages in the connective tissue subjacent to the oral epithelium contribute to the production of IL‐1 but those in connective tissue subjacent to the pocket epithelium play a different role in