We recently adopted immobilized jacalin as an affinity adsorbent to purify human serum IgA for laboratory study. In the course of our investigation, we detected a serum protein that co-eluted with IgA from jacalin-agarose affinity column. It constituted in significant quantity (24.0 +/- 0.9%, n = 30) of total jacalin-bound protein (JBP) and the yield was equivalent to 0.4 +/- 0.1 mg per ml serum. The molecular mass of this protein was 55 kDa with electromobility in the alpha 2 region as demonstrated by SDS-PAGE and immunoelectrophoresis. N-terminal microsequencing of this 55 kDa protein revealed that it is human alpha 2-HS glycoprotein (alpha 2HSG). The molecular interaction of alpha 2HSG with jacalin was characterized by competitive ELISA: human serum IgA, human colostrum secretory IgA (sIgA), and monosaccharides including D-galactose and melibiose exhibited strong inhibitory effect on its binding to jacalin. Accordingly, we propose that human alpha 2HSG binds in a similar manner as that of the bovine fetuin to jacalin. In addition, alpha 2HSG displays similar binding property to jacalin from different geographic area (India and Malaysia) and from different laboratory preparations (Sigma, Pierce and 'homemade' jacalin).