Based on ROC (receiver operating characteristic) analysis, contrast perception by eight physicians engaged in x-ray diagnostics for different lengths of time was examined in respect of inversion of grey scale values of digital x-ray film images. The ROC analysis showed a significantly superior contrast perception for imaging by the negative mode (bones white) than by the positive mode (bones black). With r* = + 0.61 a positive correlation was obtained between the period of the physicians' activity in x-ray diagnostics and the difference between the areas below the ROC curves for both imaging modes. Hence, the negative mode is to be preferred for diagnostic work based on digital x-ray film images.