Adult blowflies, Calliphora vicina Robineau-Desvoidy, are exposed to bacteria-laden decay as adults and larvae. They are protected from infections in such habitats by a family of lytic proteins called Cecropins. In this study, we explore the relationship between the developmental stages of the blowfly and the strength of the immune response when insulted by an endogenous pathogen, Escherichia coli. Our data indicate that the protective function of Cecropin B varies with developmental stage. Pupae that were not fully tanned exhibited the greatest response to an insult by E. coli. The expression of an increased immune response is due in part to an increased concentration of Cecropin B manufactured by this instar.