Summary. Aim: To study the peculiarities of ecological relationships of breast cancer (BC) cell lines MCF-7, BT-474 and MDA-MD-231 under co-culturing conditions. Materials and Methods: Three BC cell lines: luminal A — MCF-7, luminal B — BT-474 and triple-negative — MDA-MD-231 were co-cultured pairwise. Immunocytochemistry was used to differentiate the cell lines in the wells. The effect of the cell-free culture medium on the growth rate of the alternate cell line in the pair was also evaluated. Results: It was shown that when BT-474 cells were co-cultured with MCF-7 and BT-474 cells were co-cultured with MDA-MD-231, two types of ecological interactions could be observed: commensalism and amensalism, respectively. While the cells do not interact with each other in contact, the supernatants of single cultures of MCF-7 and MDA-MD-231 exert the same effect on BT-474 as co-cultivation of BT-474 with these cells. Conclusions: The paracrine mechanism of intercellular interaction between different human BC cell lines has been demonstrated. The models used in population ecology can be applicable to identify the types of interaction between cell lines.