Some information about the lactate dehydrogenase NAD binding site has been obtained by working with coenzymes analogs of incomplete molecules. 5'AMP, 5'-ADP, ATP, 5'-c-AMP and 3'(2)-AMP inhibit chicken liver LDH activity competitively with NADH. 5"-AMP and 5'-ADP show a stronger inhibition power than ATP, suggesting that the presence of one or two phosphate groups at the 5' position of adenosine, is essential for the binding of the coenzyme analogs at the enzyme binding site. Ribose and ribose-5'-P do not appear to inhibit the LDH activity, proving that purine base lacking mononucleotides do not bind to the enzyme. 5"-ADPG inhibits LDH activity in the exactly as 5'-ADP, showing that ribose moiety may be replaced by glucose, without considerable effects on the coenzyme analog binding. 2'-desoxidenosin-5'-phosphate proves to be a poorer inhibitor of the LDH activity than 5'-AMP, indicating that an interaction between the--OH groups and the amino-acids of the LDH active center takes place. Nicotinamide does not produce any inhibition effect, while NMN and CMP induce a much weaker inhibition than the adenine analogues, thus indicating a lesser binding capacity to the enzyme. Therefore, the LDH binding site seems to show some definite specificity towards the adenina groups of the coenzyme.