Histone acetyltransferases from the ribosomes of rabbit reticulocytes were purified more than 100 fold. The procedures of purification included the following: acetyltransferases were released from ribosomes by 0.5 mol/L KCl treatment, then isolated by sedimentation through buffered sucrose containing 0.5 mol/L KCl. They were partially purified from the high salt washing fraction by sequential chromatography on histone-sepharose 4B, DEAE-cellulose 52 and phosphocellulose P11 columns and by nondenaturing polyacrylamide gel electrophoresis. Three main active bands were observed on 7.5% polyacrylamide gels under non-denaturing conditions with molecular weights of about 120,000, 70,000 and 40,000 respectively. The optimal pH for acetyltransferase activity was 7.5-8.0, and 1 mmol/L MgCl2 and 25 mmol/L KCl were required for activity. 70% of the activity was inhibited by 100 mmol/L MgCl2. The Michealis constants derived from double-reciprocal plots of the acetyltransferases for histone and acetyl coenzyme A were 1.2 mumol/L and 18-20 mumol/L respectively.