The paper reviews efforts to establish Polish regulations for Diesel exhaust smoke levels. Two particular types of tests were investigated. The first, the acceleration method, evaluated smoke concentrations by absorption methods during unloaded (disengaged) engine acceleration. The second, the loading method, evaluated by filtering techniques concentrations during braking from full engine speed in driving trials.The testing procedures and accuracy of these two methods are compared. The acceleration method, although simple and quickly administered, requires precise absorption measuring equipment and does not indicate the true level of smoke of engines under loading. The accuracy of the results is dependent upon the manner of measurement and correlations between unloaded and loaded engine operation. The loading method uses filtering devices and accordingly requires more work and expense than the acceleration method. However, it allows direct evaluation of loaded engine operation and, given the lack of proper absorption apparatus, it was recommended as the official testing procedure. Subsequently, the effect of various factors on the accuracy of such testing is discussed.