Human erythrocytes were loaded with homogeneous acetaldehyde dehydrogenase (AcDH) purified from Alcaligenes Eutrophus (an enzyme species with an apparent Km for acetaldehyde similar to the mitochondrial enzyme), using an encapsulation procedure based on hypotonic haemolysis, isotonic resealing and reannealing. The AcDH-overloaded erythrocytes contained 1.55 +/- 0.25 I.U. of AcDH activity per ml of packed erythrocytes, a value 12-15 times higher than that of corresponding unloaded or native red cells. The AcDH-loaded erythrocytes were found to metabolize 4 +/- 0.8 mumol of acetaldehyde/hr/ml of red blood cells, whereas the glycolytic activity was almost unmodified. Estimates of intracellular adenine nucleotides showed 50% ATP decay in the AcDH-loaded cells when incubated in the presence of acetaldehyde concentrations higher than 50 microM, whereas the [NAD+]/[NADH] ratio was strongly decreased but to the same extent as in control cells, suggesting that this was due to the acetaldehyde itself and not to the presence of encapsulated AcDH. Similar results were obtained using mouse erythrocytes. AcDH-overloaded mouse red blood cells from donor animals were also injected intraperitoneally into compatible recipients (Balb/C) and 80 to 85% of these were found to enter into circulation within 24 hr and to circulate with a half-life of 6-7.3 days (normal half-life 11 days). Following an acute dose of ethanol (2g/kg intraperitoneally), blood levels of acetaldehyde were significantly lower in mice receiving the AcDH-loaded erythrocytes than in controls. Blood levels of ethanol were also lower in the treated mice compared to controls. These results show that AcDH-overloaded erythrocytes can perform in vitro and in vivo as bioreactors improving alcohol and acetaldehyde metabolism, and suggest that administration of these cells to alcoholic patients could be of value in restoring to normal, or improving, alcohol and acetaldehyde metabolism.