It is known that the circadian rhythm of melatonin production depends on the intensity of illumination. Violation of the light regime leads to suppression of melatonin synthesis and the development of desynchronosis, which increases the risk of developing a number of pathologies. In this regard, it is relevant to search for opportunities to restore disturbed circadian rhythms and, especially, to correct immune dysfunctions that occur in these situations. The aim of this study was to examine the effect of a complex of melatonin, aluminum oxide and polymethylsiloxane on the lymphocytes of the spleen of mice kept under round-the-clock lighting. Materials and methods . Mice of the C57Bl/6J line were kept under round-the-clock lighting for 14 days, against which they were intragastrically injected with distilled water, aluminum oxide with polydimethylsiloxane, melatonin and a complex of melatonin, aluminum oxide and polymethylsiloxane (a new drug developed by the Research Institute of Clinical and Experimental Lymphology – Branch of the Federal Research Center Institute of Cytology and Genetics SB RAS; Patent of Russian Federation No. 2577580, 2016), represented by a complex of porous material (aluminum oxide with polydimethylsiloxane) and melatonin, immobilized in the pores, from which it is gradually released in a liquid medium. Intact animals kept under the light regime of ST 12/12 and under round-the-clock lighting served as a control. Immunophenotyping of spleen B- and T-lymphocytes was performed on a flow cytofluorimeter with monoclonal antibodies APC CD3 and FITC CD19. For studying the distribution of cells by stages of the cell cycle in splenocytes, the amount of intracellular DNA was measured by the level of inclusion of propidium iodide. Results. Flow cytometry of the distribution of B- and T-lymphocytes of the spleen in male mice of the C57Bl/6J line kept under round-the-clock lighting conditions (KO 24/0 h) revealed a decrease in the percentage of B-lymphocytes and an increase in the number of T-lymphocytes, compared with animals kept under standard lighting conditions (the light/dark photoperiod – 14/10 hours). The ratio of CD19+/CD3+ lymphocytes of the spleen in mice under the conditions of KO significantly decreases (1.5 times) compared to intact animals (p ≤ 0.001). The administration of pure and modified melatonin (Complex M) to animals kept under round-the-clock lighting conditions has an equally pronounced normalizing effect on the cellular composition of B- (CD19) and T- (CD3) lymphocytes of the spleen, bringing the values of the studied parameters to the control values of the intact animals (p ≤ 0.001) Round-the-clock lighting affects the proliferative potential of splenocytes, reducing the number of cells in the G2/M phase, compared with animals treated with melatonin (p ≤ 0.050). The introduction of melatonin leads to an increase in the percentage of cells in the G2/M phase relative to the placebo group (p ≤ 0.050). In the group of mice treated with Complex M, the greatest increase in cells at the S + G2/M phases and the highest percentage of cells at the G2/M phase were revealed compared to the placebo control group (p ≤ 0.050). Conclusion. The complex of melatonin, aluminum oxide and polymethylsiloxane has additional immunotropic properties in relation to the modifier molecule, which, apparently, are due to the joint immunostimulating effect of melatonin and the lymphostimulating effect of the sorbent. Melatonin in the composition of the complex shows its properties more stably.