The involvement of the fat body and hemocytes in antimicrobial peptide synthesis in insects is widely acknowledged, although this idea is solely based upon indirect evidence, namely, changes in the amount of immunocytic mRNA that encodes certain antimicrobial peptides. However, PCR is not an appropriate method for testing the ability of cells to release the peptide of interest. The approach adopted in the present work was fundamentally different and implied long-term culturing of terminally differentiated immunocytes and subsequent detection of antimicrobial components in the culture medium. Our results confirm that the fat body and hemocytes are involved in the development of the antimicrobial peptide complex of blue blowfly (Calliphora vicina) larvae. The fat body and hemocytes are shown to synthesize in vitro defensin, cecropin, diptericins, P-peptides, and unidentified substances with bacteriostatic activity. The spectra of peptide antibiotics released by the fat body and hemocytes partially overlap. The