This paper describes an experimental program testing facility called the interactive semantic modelling system (ISMS). The ISMS is designed to allow experimentation with a wide variety of tools for collecting, analyzing, and displaying testing information. The design methodology is applicable to procedural programming languages, and Algol 60 is being used as the vehicle for elaboration of design principles and implementation techniques. This paper discusses the ISMS design, and describes the various types of analysis and display tools being developed to facilitate program testing. The ISM Preprocessor is described, an an example is presented to illustrate the data structures utilized in the ISMS.