A few architectural design studios made a particular contribution to mass housing construction in Belgrade in the second half of the 20th century. The focus of the relevant studies and articles has largely been on the output of the studios belonging to the so-called 'Belgrade school of housing construction', and, to a somewhat lesser extent, on major construction companies as protagonists of modernism. However, the work of the Research Studio (Biro za studije) which operated within Rad Construction Company has been less written about and, consequently, is less known. The Studio was founded as experimental, for research and development of prefabricated products for industrialized mass construction. Among the projects in which its role was central are: Experimental Blocks 1 and 2 in New Belgrade, the eight-storey residential buildings in 27 Marta Street in Belgrade, an experimental building in Krnjača, and the elaboration of architect Ivan Antić's design for six residential high-rises in Zvezdara. Through analyzing two important residential complexes, one in New Belgrade, the other in the old part of the city, the paper examines relations in mass housing construction and in the architecture of modernist blocks: relations between designers and large systems and relations between other actors involved in construction. In conclusion, the paper suggests a few guidelines and discusses the heritage potential of the analyzed buildings.