The action of the 17-member azalide 4'-demycarosyl-20-deoxo-20-(di-N-benzylamino)-8a-aza-8a-homotyl osin was examined in vitro using five different human cell lines: laryngeal carcinoma (Hep2), pancreatic carcinoma (MiaPaCa2), breast carcinoma (MCF7), neuroblastoma, and normal diploid fibroblast (Hef522). After exposure, the cell growth was arrested, and morphological changes occurred in a dose-dependent manner. At a concentration of 10(-4) M the azalide completely inhibited the growth of all cell lines examined and induced morphological changes such as cell shrinkage, chromatin condensation, and DNA fragmentation. These features point to apoptosis.