An acetyl-CoA hydrolase that is labile at low temperature was purified to homogeneity from the supernatant fraction of rat liver. The monomeric molecule, estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, had a molecular weight of about 63 000, while that of the purified enzyme, estimated by gel filtration, was 135 000. Thus, the enzyme consists of two subunits of identical molecular weight. On addition of adenosine 5'-triphosphate (ATP) or adenosine 5'-diphosphate (ADP) at 25 degrees C, the dimeric form of the enzyme aggregated to tetrameric forms (Mr 242 000 and Mr 230 000, respectively), whereas addition of adenosine 5'-monophosphate had little effect on enzyme association (Mr 145 000). When ATP was removed from the ATP-treated tetrameric enzyme by dialysis, the tetramer was mostly dissociated into the dimeric form. The apparent Km values for acetyl coenzyme A of the dimeric enzyme and tetrameric enzyme, reconstituted from the former in the presence of 2 mM ATP, were 170 microM and 60 microM, respectively. The purified dimeric enzyme was inactivated by exposure to lower temperature, especially below 10 degrees C. The various nucleotides tested partially stabilize the dimeric enzyme at low temperature, ATP being the most effective. Sucrose density gradient centrifugation showed that loss of catalytic activity by cold treatment was accompanied by dissociation of the dimer and tetramer into protomer.