The effects of butyric acid on cloned human lung cancer cells (PLA-801D) were observed with flow cytometry (FCM) and the conventional methods to identify the biological characteristics of cell lines. Butyric acid ( 2 or 3 mmol/L)treatment could result in an inhibition of the cell division and the growth rate of the cells,the blocking of the cell cycle progression in the Gl phase,a prolongation in doubling time, a shortening in saturation time, a decrease of saturation densities,and a reduction of plating efficiency. At the same time,there were some normal phenotypes of the cells such as flattening, tendency to grow in a monolayer,weakening of surface membrane activities, regularization of nuclei, and increasing of intercellular junctions.These findings suggest that after the treatment of butyric acid, the cancer cells (PLA-801D) appeared to acquire relatively normal growth properties and phenotypes. Thus, butyric acid, to a certain extent, can induce differentiation of PLA-801D cells, and be a potential anticancer agent.