Following exposure of rats to the arylamine carcinogen 2-aminofluorene, DNA-carcinogen adducts were found in the liver and bladder target tissues, and also in circulating leucocytes. This work investigated the effect of ellagic acid on arylamine (2-aminofluorene and p-aminobenzoic acid) acetylations in rat leucocytes. Evidence is presented that rat mononuclear leucocytes are capable of acetylating 2-aminofluorene and p-aminobenzoic acid. Both lymphocytes and monocytes were able to acetylate arylamines during 18 h of culture. Cultured lymphocytes produced about twice as much N-acetyl-2-aminofluorene from 2-aminofluorene and 2.2-fold as much N-acetyl-p-aminobenzoic acid from p-aminobenzoic acid as monocytes. After cotreatment with ellagic acid the lymphocyte and monocyte cultures indicated that ellagic acid reduced 2-aminofluorene acetylation.