Considerable effort is being made to design anti-viral drugs for the human immunodeficiency virus type 1 (HIV-1) infection process. Some of this work has focused on CD4 protein, the HIV-1 receptor on T helper lymphocytes. One drug that binds to CD4 protein and inhibits both viral infection and growth is DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonate). DIDS is best known for its ability to inhibit erythrocyte band 3 anion exchange. Although the antiviral potency of DIDS is evident in vitro (IC50 approximately 30 microM), intravenous administration of DIDS should not be effective owing to the large number of band 3 molecules present on the red blood cell membrane (approximately 10(6)/cell), and to the very small Kd for DIDS binding to band 3 (approximately 30 nM). Therefore, we sought to identify other anion transport inhibitors that would bind weakly to band 3, but tightly to CD4 protein, and that could be administered to humans without significant toxic side effects. On the basis of our previous work with band 3 (Salhany, J. M., Rauenbuehler, P. B., and Sloan, R. L. (1987) J. Biol. Chem. 262, 15965-15973), we elected to study the binding of pyridoxal 5'-phosphate (PLP) to soluble CD4 protein. We have discovered that PLP binds surprisingly tightly to soluble CD4 protein (Kd = 45 microM), with a stoichiometry of about 1 mol of PLP/mol of protein. Furthermore, PLP binding was found to be competitive with DIDS for its binding site on soluble CD4 protein. These results suggest that PLP may be an effective anti-viral agent for the HIV-1 infection process.