After treating human lung adenocarcinoma cells by Na2SeO3 (1 microgram/ml) for 24 hours, its mitotic index decreased by 50%. The reduction rate was directly proportional to selenite concentration (1-5 micrograms/ml). In the meantime, the labelling index, growth rate and progression of the cell cycle were also inhibited. In contrast to the above observation, there was no marked change in the cell count, mitotic index, labelling index and the average silver granule number in the human embryonic lung diploid cells treated by Na2SeO3 (1-5 micrograms/ml) for 1-3 days. When human embryonic lung diploid cells were mix-cultured with human lung adenocarcinoma cells in the presence of 5 micrograms/ml Na2SeO3 for 24 hours, the former maintained a normal morphology, while the lung cancer cells showed heavily vacuolated cytoplasms and distorted nuclei.