During the course of their recirculation through the body, blood-borne 
lymphocytes specifically adhere to high endothelial venules (HEV) within 
secondary lymphoid organs such as peripheral lymph nodes (PN) and gut-associated 
Peyer's patches (PP). This adherence event, which initiates the extravasation of 
the lymphocyte, is highly specific in terms of the class of lymphocyte and the 
anatomic location of the HEV. We review evidence that the lymphocyte adhesive 
molecule ('homing receptor') involved in attachment to PN HEV is a 
carbohydrate-binding receptor (lectin-like) with specificity for 
mannose-6-phosphate (M6P)-like ligands. We describe the use of a novel 
cytochemical probe for the detection and characterization of cell surface 
carbohydrate-binding receptors. Using a M6P-based probe, we show that the 
carbohydrate-binding receptor on lymphocytes is closely-related or identical to 
the MEL-14 antigen, a putative homing receptor identified by a monoclonal 
antibody. Evidence is presented that the lymphocyte attachment sites on both PN 
and PP HEV are inactivated by mild periodate oxidation and hence are probably 
carbohydrate in nature. Yet, the sites are biochemically distinguishable in that 
one class (PN) requires sialidase-sensitive structures whereas the other (PP) 
does not. We raise the possibility that diversity in the carbohydrate-based 
recognition determinants on HEV may underlie the adhesive specificities in this 
system.