Human lactate dehydrogenases (LDH-A₄, -B₄, and -C₄) are highly homologous with 84–89% sequence similarities and 69–75% amino acid identities. Active site residues are especially conserved. Gossypol, a natural product from cotton seed, is a non-selective competitive inhibitor of NADH binding to LDH, with K_i values of 1.9, 1.4, and 4.2 μM for LDH-A_4, -B_4, and -C_4, respectively. However, derivatives of gossypol and structural analogs of gossypol in the substituted 2,3-dihydroxy-1-naphthoic acid family exhibited markedly greater selectivity and, in many cases, greater potency. For gossypol derivatives, greater than 35-fold selectivity was observed. For dihydroxynaphthoic acids with substituents at the 4- and 7-positions, greater than 200-fold selectivity was observed. Inhibition was consistently competitive with the binding of NADH, with dissociation constants as low as 30 nM. By comparison, a series of N-substituted oxamic acids, which are competitive inhibitors of the binding of pyruvate to LDH, exhibited very modest selectivity. These results suggest that substituted dihydroxynaphthoic acids are good lead compounds for the development of selective LDH inhibitors. Selective inhibitors of LDH-C₄ targeted to the dinucleotide fold may hold promise as male antifertility drugs. Selective inhibitors of LDH-A₄ and -B₄ may be useful for studies of lactic acidemia associated with ischemic events. More broadly, the results raise the question of the general utility of drug design targeted at the dinucleotide binding sites of dehydrogenases/reductases.