The review summarizes the results of studying the space and simulated microgravity effects on cell-to-cell interactions in three models: 1) interaction of a human lymphocytes culture with a subinoculated suspension culture of tumor myeloblasts K-562; 2) formation of embryoid bodies and differentiation of mice embryonic stem cells (ESCs); 3) gene differentiation and expression in multipotent mesenchymal cells (MCSs) obtained from the human marrow. It was shown that microgravity modifies the cell-to-cell interactions without cell contact inhibition; however, micro-g causes reversible changes in expression of genes responsible for regulation of the cytoskeleton, focal adhesion proteins, and some cytokines. Prolonged exposure of progenitor cells in simulated microgravity inhibits the differentiation processes of and reprofiles significantly expression of the genes that control various cell activities, including cell-to-cell interactions.