The kinetics of the interaction between the affinity label 5'-p-fluorosulfonylbenzoyladenosine (FSBA) and the 'high Km' form of aldehyde reductase have been studied. The results of this investigation indicate the reaction to fulfill two of the major criteria of true affinity labelling (see Colman 1983), namely, that the reaction follows saturation kinetics, indicating the initial formation of a reversible enzyme-inhibitor complex, analagous to the enzyme-substrate complex, prior to the irreversible modification step, and that the interaction may be prevented specifically by the ligand (coenzyme in this case) which normally occupies the site towards which the label is directed. The amino acid(s) which binds covalently FSBA would thus be expected to be present within the coenzyme binding site of aldehyde reductase. The identification of the residue(s) attacked by this compound is currently under investigation.