The NAD analogue [3-(3-acetylpyridinio)-propyl]adenosine pyrophosphate forms enzymically inactive complexes with glyceraldehyde-3-phosphate dehydrogemase from yeast and rabbit skeletal muscle. In the latter enzyme four mol of the analogue are bound with equal affinity inhibiting the enzyme in a competitive way: K1 = 0.0 mM as compared to the dissociation constant KD = 0.6 mM. The brominated derivative [3-(3-bromoacetylpyridinio)-propyl]adenosine pyrophosphate is covalently bound to both enzymes causing irreversible loss of enzymic activity. Complete inactivation of the enzyme from muscle required two moles of the analogue per mol of tetramer. The remaining two sites are still able to bind two mol of NAD+ without regain of enzymic activity. In the case of the yeast enzyme four mol of the analogue are bound. Inactivation of the rabbit muscle enzyme is accompanied by the disappearance of two out of four highly reactive sulfhydryl groups; in the yeast enzyme the four active site cysteine residues are still able to react with DTNB, the reactivity being diminished significantly. Hybrid formation between the native enzymes from yeast and skeletal muscle is not affected by the modification of the enzyme. Similarly the sedimentation properties of the covalently modified enzyme are indistinguishable from those of the native molecule. This indicates that both the native and the irreversibly inhibited enzyme are identical regarding their quaternary structure.