Human alpha-defensins contribute to local intestinal host defense as part of innate immunity and may be of major relevance in microbial infection and chronic inflammatory bowel disease. The objective was to investigate human defensin 5 and 6 (HD-5, HD-6) mRNA expression by RT-PCR from colonic biopsies and peptide expression by immunohistochemistry from colonic resections under basal and inflammatory conditions. Both alpha-defensins were induced by proinflammatory cytokines in cell culture. HD-5 mRNA expression was enhanced in both idiopathic and nonidiopathic inflammatory states of the large bowel [32% of control vs 73% of unspecific colitis, 69% of Crohn's disease (CD) and 73% of ulcerative colitis (UC)], whereas HD-6 was specifically related to CD and UC (14% in controls vs 20% in unspecific coltis, 49% in CD and 42% in UC). Immunohistochemistry confirmed the presence of HD-5 in colonic epithelium. Antimicrobial peptides in the colon may be of importance in maintaining the mucosal barrier and controlling microbial invasion in inflammatory bowel diseases.