The effects of the walls of a test section on a model in transonic flow were investigated by using the AGARD Calibration Model B. Tests were carried out in a closed-circuit pressurized tunnel, with a confined square test section of 1.5 m width, with tapered slots giving a 5% porosity. Two models with different dimensions were used, with 0.85 and 0.056% blockage ratios. Longitudinal aerodynamic characteristics were analyzed by means of measurements performed at varying angles of attack (up to 24 deg) and Mach numbers from 0.3 to 1.2. In some flow conditions wall interference effects were probably present. However, the forces and moments dependent on the pressure distribution were likely to be related to the same factors, and therefore, the above effects tended to disappear when longitudinal stability and lift-dependent drag were analyzed as a function of lift characteristics. The drag rise Mach number evaluation seems be fully free from blockage effects. The dimensions of the tested larger model can be considered to be the largest reasonable ones for industrial applications, but, probably, not sufficiently small when high accuracy is required.