We studied the development of mouse embryos in vitro depending on the number of embryos in a given microvolume of the Ham's F-10 medium without protein or with the addition of serum. The absence of serum from the culture medium did not affect the development of two-cell embryos cultivate in groups of 5-6 (about 90% embryos developed until the stage of blastocyst and over 50% left zona pellucida), but the development of single embryos in the protein-free medium proceeded significantly worse. Single two-cell embryos cultivated in the serum-containing medium developed similar to embryos cultivated in groups. At the same time, no significant differences in development was found between eight-cell embryos cultivated individually or in groups of up to 10 embryos in the Ham's F-10 medium, either in the presence of serum or without it (about 95% embryos developed until the stage of blastocyst and over 70% left zona pellucida). The increase in the number of cultivated embryos over 10 had the adverse effect on development of either two-cell or eight-cell embryos. The attachment of blastocysts to the substrate after leaving zona pellucida and growth of trophectoderm was observed only in the presence of serum. These results suggest that interaction between preimplantation embryos in culture can probably be mediated by factor(s) released by embryos into the medium. Serum appears to contain such factor(s). Sensitivity of embryos to the factor(s) apparently depends on the stage of development.