Aliphatic polyamines, putrescine, spermine and spermidine present in bacteria and eukaryotic cells are essential for cell growth. Generally, polyamine levels are elevated in rapidly growing normal and pathological systems. Since polyamines belong to the category of molecules whose synthesis is strongly activated during the G1 period they have been implicated in the cell's preparation for DNA replication. In our experiments, we have differentiated by phorbol-ester, the human pre-B leukemic cell line KM-3. Biochemical and flow cytometric analysis show an increase, in treated cells, of polyamines pathway related to G1 and early S-phase of cell cycle during B cell differentiation.