The High Level Architecture (HLA) is a simulation infrastructure designed to promote interoperability between simulations. The Defense Modeling and Simulation Office (DMSO) organized several experimental applications of HLA in 1996 to test and refine the architecture. One of those experiments was performed by the Platform Proto-Federation (PPF), a group of virtual real-time (i.e. DIS-type) simulations assembled to test HLA applicability to that domain. The PPF consisted of four member programs: BDS-D, BFTT, CCTT, and JTCTS. Each member program implemented an HLA federate that simulated one or more combat entity objects and interoperated with the others via HLA. That interoperation was tested in an experimental scenario that included land, sea, and air entities and a range of combat interactions. The PPF experiment showed that HLA provides the requisite functionality to support DIS-type distributed simulation, and is a promising component of future simulation development, but the run-time performance of production HLA software must be substantially improved over that of the prototype HLA software used for the PPF experiment.