The lectin (PHA) from Phaseolus vulgaris is a tetrameric glycoprotein composed of two different subunits (E and L). The lectin exists as five isolectins (L4, L3E, L2E2, LE3, and E4) that apparently originate from all possible tetrameric combinations of the two subunits. We have investigated two isolectins, PHA-E4 and PHA-L4, with respect to their affinity for human serum glycoproteins, and have subsequently determined how these interactions modulate the ability of the isolectins to induce lymphocyte transformation. PHA-E4 has affinity for 14 identified serum glycoproteins, whereas PHA-L4 exhibited affinity for nine of these proteins. IgM, IgA, IgG, alpha 2-macroglobulin, beta-lipoprotein, and haptoglobin showed the strongest reactivity with PHA-E4; haptoglobin, IgG, and IgA were also the best ligands for PHA-L4. The glycoprotein levels of human serum were then reduced by running the serum over PHA-E4 and PHA-L4 affinity columns, respectively, and the glycoproteins that bound to the two immobilized isolectins were eluted, pooled, and concentrated. The ability of PHA-E4 and PHA-L4 to activate human lymphocytes in the presence of (i) normal human serum, (ii) human serum with reduced levels of glycoproteins, and (iii) human serum with elevated levels of glycoproteins revealed a strong regulatory effect of the glycoproteins on the lymphocyte transformation. When the inhibitory glycoproteins and glycolipids were removed from serum, the mitogenic effect of PHA-E4 increased 200-fold, whereas PHA-L4 exhibited only a 25-fold increase in mitogenic activity. These results show that the so-called "erythroreactive isolectin (PHA-E4)" actually possesses a strong mitogenic activity of its own, but that this has been masked by the interference of serum components.