A method preserving the activity of the erythrocytic enzyme delta-aminolevulinic acid dehydratase (EC 4.2.1.24) was developed using a vehicle of 50% aqueous glycerol containing dithiothreitol (80 microM). Whole heparinized blood (0.5 ml) was added to 0.75 ml of this mixture in a cryovial tube (capacity 1.3 ml), mixed well and stored in a freezer at -20 degrees C for 21 days. Statistical comparison of the enzyme activity when freshly assayed and after storage indicated excellent agreement for quantitation of the non-activated enzyme (intraclass correlation coefficient = 0.99) and the activated enzyme (intraclass correlation coefficient = 0.98). This storage method will facilitate future population studies on lead intoxication, particularly those in remote locations.