Aldehyde dehydrogenase has the ability to catalyse the hydrolysis of p-nitrophenyl esters as well as the dehydrogenation of aldehydes by NAD+. Some work has been interpreted in terms of two different types of active site (Blackwell et al., 1983), but much other evidence points to the identity of the dehydrogenase and esterase sites (Loomes and Kitson, 1986; Kitson et al., 1991). In particular, Cys-302 has been identified as the nucleophile that becomes acylated by the aldehyde substrate trans-4-(N,N-dimethylamino)cinnamaldehyde (Pietruszko et al., 1993) and by the ester substrate analogue p-nitrophenyl dimethylcar-bamate (Kitson et al., 1991). Thus studying the esterase activity of aldehyde dehydrogenase should be capable of giving information about the active site of relevance to the dehydrogenase activity.