Inoculation of live Escherichia coli into tsetse flies, Glossina morsitans morsitans, stimulated a higher antibacterial immune response in females than in males. It increased with age in females from emergence to approximately 2 weeks and thereafter declined. In males, there was also a significant decrease in immune response with aging. Inoculation of killed bacteria failed to stimulate antibacterial activity but stimulated a lysozyme response which was weaker than that stimulated by live bacteria. No antibacterial activity was present in the hemolymph of larvae from immunized pregnant tsetse. Inoculation of live Trypanosoma brucei brucei and T. congolense failed to induce production of antibacterial activity and lysozyme. Furthermore, tsetse inoculated with or naturally infected with T. b. brucei and T. congolense failed to show any evidence of immunosuppression when challenged with live E. coli. Various species of live bacteria stimulated different levels of antibacterial factors, with Enterobacter cloacae stimulating the highest level of antibacterial activity and E. coli the highest level of lysozyme. Saline in which certain species of bacteria and T. b. brucei were incubated inactivated tsetse immune hemolymph.