1. The characterization and partial purification of an induced lysozyme activity in the hemolymph of adult Rhodnius prolixus inoculated with Micrococcus lysodeikticus is described. 2. Little or no activity against M. lysodeikticus appeared in the first hours after inoculation, but the activity increased reaching a maximum 4 days later, which was maintained to day 12. 3. The activity was characterized as lysozyme on the basis of the following considerations: 1) pH optimum and thermostability at acidic pH; 2) rate of lysis negatively dependent on ionic strength; 3) binding to SP-Sephadex at pH 5.5; 4) apparent molecular weight of 15 kDal. 4. Crude or semi-purified enzyme preparations showed a high degree of stability during handling, freezing and thawing, and standing at 5 degrees C. 5. Incubation of abdominal fat bodies from treated insects resulted in the release of activity into the medium. 6. The relationship between induced lysozyme activity and its role as an insect defense mechanism is discussed.