The Montgomery County, Maryland Public Schools, working with Operations Research, Incorporated, developed a Pupil Master File (PLMF). PLMF is one of the several applications included within the Pupil Subsystem of the district's overall computerized School Information System. The PLMF is designed to provide: 1) a master file which enables users to implement the pupil data system; and 2) interface capability with other subsystems. It protects the confidentiality of sensitive data, provides retrieval access to the entire system, and facilitates input, output, and file expansion. PLMF uses COBOL software and is implemented on IBM S/370 equipment. The application is used to measure educational progress, to supply data for planning and projections, to generate data for research purposes, and to supply teachers with pupil profiles to aid classroom decision-making. Since PLMF ties several applications of the Pupil Subsystem together, thereby reducing errors and increasing efficiency, it is judged a useful tool for the educational administrator. (PB)