Fibrotic lesions accompany several pathological conditions including tumors . We show that expression of a dominant-active form of the Ras oncogene in Drosophila salivary glands (SGs) leads to redistribution of components of the basement membrane (BM) and fibrotic lesions . Similar to several types of mammalian fibrosis, the disturbed BM attracts clot components including insect transglutaminase and phenoloxidase . SG epithelial cells show reduced apico-basal polarity accompanied by a loss of secretory activity . Both the fibrotic lesions and the reduced cell polarity are alleviated by ectopic expression of the antimicrobial peptide Drosomycin (Drs), which also restores secretory activity of the SGs . In addition to ECM components, both Drs and F-actin localize to fibrotic lesions.