Inducible cecropin-like and attacin-like proteins were isolated from immune hemolymph obtained from vaccinated Heliothis virescens larvae. The attacin-like protein had a molecular weight of approximately 25,000 daltons and was not dialyzable. The cecropin-like peptide had an estimated molecular weight of 6,000-7,000 daltons and was dialyzable, heat-stable and sensitive to trypsin digestion. The cecropin-like peptide showed bactericidal activity against Escherichia coli and Enterobacter cloacae, and the attacin-like protein showed bactericidal activity against E. coli. The immune hemolymph was bactericidal against E. coli, E. cloacae and Pseudomonas aeruginosa. Ultrastructural cell envelope damage to E. coli, produced by the immune hemolymph, was observed by scanning electron microscopy. No antiviral activity by the inducible cecropin-like and attacin-like proteins was detected against herpes simplex virus-1 and the vesicular stomatitis virus.