In a highly differentiated Morris hepatoma 5123D the fraction of repetitive, cytoplasmic poly(A)-containing RNA hybridizing with DNA to the Cot values below 10(2) mol X sec/l is present in a higher proportion than in normal rat liver. 2. Unique sequences of cytoplasmic RNA containing poly(A) both in Morris hepatoma and normal rat liver exhibit similar hybridization kinetics. 3. No sequence differences in cytoplasmic poly(A)-free RNA between Morris hepatoma 5123D and normal rat liver were found using hybridization-competition technique.