Melatonin, as an endogenous hormone, is associated with the moderation of the circadian rhythm. Besides controlling the sleep-wake cycle, many physiological functions of melatonin have been identified, such as antioxidant, immune modulating, and anti-inflammatory. The current study was designed to estimate the effect of different doses of melatonin on gene expression of (TNF) in serum and mitotic index (MI) in bone marrow stem cells as cytogenetic test. This study was conducted at Wasit University, College of Science, Department of Biology, Cytogenetic Laboratory, from the period 2/11/2021 to 14/4/2022 using albino mice. The mice were divided into four groups, a control group and three groups divided according to melatonin concentration (5, 10, &15 mg/kg), and were treated for seven consecutive days for every dose. Results showed a significant decrease in gene expression (protein) of TNF α- at P≤0.05 for all three doses of melatonin that were used and mitotic index numbers increased at P≤0.05 for the same doses above of melatonin, when all results above were compared with the control group. 
