Over the past four decades, a large number of composite prestressed concrete beam/slab bridges have been built in North America. To evaluate the load rating of these bridges, an assumption must be made concerning the existing effective prestress force. This assumption is difficult to make because the effective prestress force is influenced by several time-dependent phenomena such as shrinkage and creep of the concrete and relaxation of the prestressing strands. This paper presents the results of an experimental study to determine the effective prestress force in two fullscale prestressed concrete bridge beams that were removed from a bridge structure after a period of 28 years in service. An average prestress loss of 18 percent was determined for the two specimens. This loss value is approximately 60 percent of the loss predicted according to design specifications.