A lectin with potent mitogenic activity was isolated from Musca domestica pupae. The lectin was a monomer with a molecular weight of 55 kDa. The purification procedure mainly involved affinity chromatography on galactose-Sepharose-4B. The lectin agglutinated trypsin-treated rabbit blood cells. D-galactose was detected to inhibit the lectin's hemagglutination activity. The lectin was metal independent and temperature dependent and was detected by the periodic acid-Shiff reaction to be a glycoprotein with a carbohydrate content of 2.1%. Observations with an atomic force microscope indicated that the lectin was a globular protein about 75 nm in size, with several carbohydrate chains linked to it. It elicited a mitogenic response from mouse splenocytes in vitro, with the maximal response at a concentration of 20 microg/ml. No lectin has been reported before from Musca domestica pupae, and the lectin we discovered could be of great importance in insect immunity and hypoimmune diseases.