One aspect in the relationship between the problems of communication and activity consists in the fact that within the theory of activity there is no place for the analysis of activity's social means and tools - and particularly semiotic tools. Proceeding from L.S. Vygotsky's and M.M. Bakhtin's works the author suggests a structural-genetical unit of activity: the social (common) act of an individual. At every moment a social act is directed to a material object (object-bound). At the same time a social act performed by an individual finishes not at the moment of acquisition of the corresponding object, but at the start of a correlated act of some other individual. Ontogenetically the social act is a point of departure. It certainly includes both communication and signs since it is directed at an "other", and has therefore to contain a sygnal for this other. In the process of interiorization the structure of the social act is getting curtailed and as a result of corresponding transformations produces fundamental social structures determining development of the whole totality of psychic processes. Finally the social act is a universal element of both individual and common activity being activity and communication to the same extent.