Although it is sympatric with tsetse flies, Stomoxys calcitrans is not a biological vector of trypanosomes. It is known that haemolymph (HL) and midgut reservoir zone (RH) lectins regulate parasitic infections in some dipteran insects. Agglutinins (lectins) were detected in HL and RH from unfed stable flies (maximum titre 2-6). Increased haemagglutination activity resulted post-feeding (maximum titre 2- 16 - 2- 18). Optimum titres varied according to agglutinogen type and mammalian blood source. Rabbit erythrocytes produced the highest haemagglutination titres followed by human group B, human group 0, horse, human group A, human group AB and sheep. Stomoxys haemagglutination activity was found to be 1.5 - 2.5 times stronger than that of Glossina. Whole blood-fed flies produced the highest titre (2-18), compared to glucose-fed insects, against rabbit erythrocytes. Anti-Trypanosoma brucei brucei titres ranged from 2-6 - 2-7 in both tissues. Similar results were obtained with Leishmania hertigi and Crithidia!asciculata. Purification of the samples was performed in order to draw conclusions with confidence regarding the physico-chemical properties of the agglutinins (lectins) and in order to determine the molecular weight of the agglutinins. Protein contents ofHL and RH samples of flies aged < 12 hours to 3 days were determined. They were 25 - 28 mg/ml and 6.4 mg/ml respectively. Protein contents increased with age reaching 32 mg/ml for HL and 7.2 mg/ml for RH at day 14 post-emergence (p.e.). The contents then started to decrease reaching 22 mg/ml for HL and 5.6 mg/ml for RIi at day 28 p.e. Purified lectins constitute 4.3% of the total protein contents in RH samples (having molecular weights of 26,302 Da, 16,218 Da and 14,028 Da) and, approximately twice, 9.47% of the total protein contents, in HL samples (having similar molecular weights of 28,300 Da, 16,218 Da and 14,600 Da). HL and RH anti-parasite and anti-erythrocyte agglutinins (lectins) were basic glycoproteins in nature, calcium ion dependent for activity, heat labile, freeze-thaw sensitive and required slightly acid to alkaline pH conditions for optimum agglutination. Lectins were specific for galactosyl and glucosyl moieties. In vivo sugar inhibition of RH lectin activity resulted in three-fold increased S. calcitrans mortalities post- T.b. brucei infection, compared to the controls, suggesting a lectin parasite-killing function. However, sugar inhibition of lectins did not lead to transformation of trypanosomes to procyclic forms or to infection of the fly.