Many ongoing selective dissemination systems designers assume that the librarian can be omitted from active participation in execution of the master plan. ISI's four years of experience with ASCA(R) service have shown that librarians must be an integral part of the system and engage in an active dialogue between users and the machine. Specific examples of how librarians can best serve the information needs of scientists using SDI systems are examined. It is the basic contention of this paper that the librarian should serve as an intermediary between users and the numerous new information media. In this manner the librarian can filter and translate the requirements of individual scientists to conform with the inherent limitations of all machine systems while exploiting their capabilities to the fullest.