Bloodstream forms of Trypanosoma congolense were cocultivated in vitro with vascular endothelial cells. The trypanosomes adhere specifically to the endothelial surfaces of the anterior part of their flagella, as shown by scanning and transmission electron microscopy. The interaction between parasite and host cell is very tight, and frequently the accumulation of endocytotic vesicles near the contact site is observed. Immunoelectron microscopy revealed a compound distributed over the total surface of the trypanosomes and reacting with antibodies against the beta 1 integrin chain, but no reaction was found with anti-alpha 1 or anti-alpha 2 antibodies. Integrins are typical adhesion molecules and are now shown to be present at the surface of T. congolense by electron microscopy and by immunofluorescence. A direct participation of this substance in the specific adhesion to endothelium, however, could not be proven. (C) 1994 Academic Press, Inc.