The substrate specificity, pH activity curves, inhibition characteristics and in vitro stabilities of the human ADH isozymes characteristic of the structural loci, ADH1, ADH2 and ADH3, have been investigated using crude tissue extracts and partially purified material. Alcohol substrates: Seventeen different alcohols were tested. The products of the three loci showed differences in their relative activities with the different substrates. Thus ADH1 isozymes were most active with ethanol, allyl alcohol, sec propanol and cyclohexanol; the 'usual' ADH2 were most active with ethanol, butanol, octanol and sec butanol; the 'atypical' ADH2 isozymes were most active with ethanol and octanol, but showed relatively low activity with butanol and Ronicol; the ADH3 isozymes were relatively very active with long straight chain primary alcohols. Aldehyde substrates: Six different aldehydes were tested. No significant differences between the isozyme products of the three loci were detected except in the case of chloral hydrate. The ADH1 and 'usual' ADH2 isozymes showed activity with chloral hydrate but this was a very poor substrate for the ADH3 and 'atypical' ADH2 isozymes. pH activity profiles: With ethanol as substrate the pH optimum for the ADH1, 'usual' ADH2 and the ADH3 isozymes was around pH 11.5 and for the 'atypical' ADH2 isozymes was about pH 8.8. With acetaldehyde as substrate the pH optima for the ADH1, 'usual' ADH2, 'atypical' ADH2 and ADH3 isozymes were about pH 8.8, 6.0, 7.0-7.5 and 6.5, resp. Inhibitors: Trichloroethanol was found to be a potent inhibitor of the ADH1 isozymes; isobutyramide an inhibitor of ADH3; and pyrazole and thiourea were shown to be powerful inhibitors of the 'atypical' ADH2 isozymes. In vitro stability: The ADH1 isozymes appeared to be relatively less stable than the 'usual' ADH2 and ADH3 isozymes. The 'atypical' ADH2 isozymes were found to be relatively very labile and particularly susceptible to freezing and thawing or storage at 10° C. The ADH 1;3 and ADH 2;3 isozymes were not demonstrably different in the properties tested.