The cecropins are a group of peptides that were first isolated from the hemolymph of the giant silk moth, Hyalophora cecropia. In preliminary studies, these novel peptides were shown to be active against several bacteria and mammalian lymphomas and leukemias in vitro. The mechanism of action of the cecropins is thought to involve pore formation at the cytoplasmic membrane. The potential anticancer activity of cecropin B, cecropin P1 and Shiva-1 was investigated against a panel of mammalian cell lines in vitro. Cell lines showed a range of sensitivities to cecropin B (IC50 3.2 to > 100 microM), and two cell lines with the multidrug-resistant phenotype were sensitive to the peptide. In vitro cecropin B activity was virtually complete within one hour. Preliminary in vivo studies showed that cecropin B increases the survival time of mice bearing murine ascitic colon adenocarcinoma cells. Future studies will address structure/activity relationships of similar peptides in order to optimize antitumor activity.