Arylhydroxamic acid N,O-acyltransferase and Co-ASAc-dependent N-acetyltransferase activities were measured simultaneously in liver cytosols from rabbits of known acetylator phenotype. Both activities were high in rapid acetylator rabbits and low in slow acetylator rabbits indicating that these two acetyl transfer steps in the metabolic activation of certain arylamines are under common genetic control in this species. The two enzyme activities could not be resolved by sequential centrifugation, fractional precipitation with ammonium sulfate, ion exchange chromatograhy on DEAE-cellulose, gel filtration on Sephacryl, and electrophoresis on polyacrylamide gels. Sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis of extracts of disc gel electrophoretic slices showed that a single symmetrical protein band with a molecular weight of 33,000 was associated with both activities. The results obtained strongly suggest that the CoASAc-dependent N-acetyltransferase reaction and the intramolecular N,O-acetyl transfer by arylhydroxamic acid N,O-acyltransferase are properties of the same enzyme.