SUMMARYIn vitrostudies of the behaviour of the trypanosomatid flagellatesTrypanosoma bruceiandLeishmania hertigiin the presence of cell-free haemolymph of locusts,Schistocerca gregariaand cockroaches,Periplaneta americanarevealed the presence of parasite agglutinins. The range of normal values of agglutination titres was 2−4to 2−13. Physico-chemical treatment of haemolymph indicated that these agglutinins are protein or glycoprotein in nature and are only partially affected by heat treatment below 65°C, at which temperature incubation of haemolymph for 30 min abrogated all agglutination. Agglutination was not dependent on the presence of Ca2+or Mg2+. Prior infection of locusts and cockroaches withT. bruceiandL. hertigisignificantly increased agglutinin titres between Days 4 and 6 in cockroaches (P< 0·05) and from Days 2 to 4 whenL. hertigiwas inoculated into locusts. The induced differences in titres observed in locusts infected withT. bruceiwere not significant. Lysozyme levels were significantly increased after inoculation ofT. bruceiinto cockroaches compared with placebo-inoculated and uninoculated controls.L. hertigiinoculation produced significant increases in lysozyme levels compared with controls between Days 1 and 7 in locusts and 3 to 6 in cockroaches. These studies indicate that, at least in easily manipulated model systems, induced responses to intrahaemocoelic inoculation to trypanosomes andLeishmaniacan occur. As far as we are aware this is the first report of an induced response of an insect to such important parasites. The possibility that induced responses in natural vectors to these parasites occurs requires investigation.