Abstract: AIM To investigate the influence of melatonin (MT) on growth of HeLa cells in vitro. METHODS The antiproliferation activities of MT were evaluated on HeLa cells by means of trypan blue dye exclusion and MTT vital staining.The morphological changes of MT on HeLa cells were observed under transmission electron microscope. Cell division cycle influenced by MT was assessed by a flow cytometer. RESULTS MT produced a certain inhibition on HeLa cells at the concentration of 2 mmol·L^-1 and prolonged the time of double amount (T_D). The fraction of cells inhibited was 61.0%. The IC₅₀ of HeLa cells exposed to MT for 96 h was 2.039 mmol·L^-1. The flow cytometric analyses showed that exposure to MT for 72 h reduced the number of HeLa cells in phase S. Under electronic microscope, the HeLa cells exposed to MT for 72 h displayed morphological changes of necrosis, apoptosis, more hetero chromosome and less somatic chromosome. CONCLUSION MT showed certain influence on HeLa cells. Its mechanism may probably be attributable to reduction of the number of cells in phase S.