Ecdysteroids are multifunctional hormones regulating virtually all morphogenetic events in insects. Their role in stress and immune response regulation is not well researched. Here we study the effect of 20-hydroxyecdysone on antimicrobial peptide synthesis in the larvae of Calliphora vicina. An inverse correlation was observed between the 20-hydroxyecdysone titer and antimicrobial peptide concentration in the hemolymph of naïve and bacteria challenged maggots. High and low doses of 20-hydroxyecdysone, injected simultaneously with bacterial cells, had an opposite effect on antimicrobial peptide synthesis in the diapausing larvae. Morphogenetically effective doses of 20-hydroxyecdysone demonstrated immunosuppressive activity. A low dose of 20-hydroxyecdysone, on the contrary, moderately stimulated antimicrobial peptide synthesis. The data suggest that ecdysteroids are directly involved in the regulation of the immune system activity and that the final effect is dose-dependant.