Characterization of activities that provide potential targets for genetic manipulation of pathogen development, maintenance, and transmission in transgenic insects has applications to the eventual control of malaria and other arthropod-transmitted diseases. We have identified inducible activities from cultured mosquito (Aedes albopictus) cells, including one that shares the antimicrobial properties of cecropins from other insects. The cecropin- like activity can be induced by treatment with heat-killed Escherichia coli, is secreted into the cell culture medium, and can be detected after electrophoresis of acid precipitable proteins on polyacrylamide gels at pH 4.3. Cecropin lacks the amino acids methionine and cysteine. Other proteins secreted in response to bacterial induction measure 111, 66, 53, and 32 kD; these proteins incorporate sulfur-containing amino acids and were detected on denaturing polyacrylamide gels. The synthesis of antimicrobial proteins by mosquito cells in culture will contribute to an understanding of the diversity of molecules that participate in insect immunity and to the use of continuous cell lines and their inducible products to explore and manipulate regulation of physiological processes relevant to vector biology.