Problems of repeated use of reference blocks for verification of penetrant testing lies in the fact that there is no exact criteria prescribed to which level it is possible to tolerate a reduced work performance of reference blocks. In practice it may happen that the reference blocks shows false indications or does not show, and that we are not even aware of, and resultant change the PT system which is correct or use the PT system that is not correct. For these reasons, it is necessary, in continuity, it is recommended once a year, check the accuracy of reference blocks for checking the PT system to reduce to a minimum such situations. Norme EN ISO 3542-2 [1] provides that should at least once a year, send a reference blocks to check accuracy by an independent laboratory, however during this period if not properly treated with established standards, may give poor quality results and prior to that deadline. Experiential give some guidance how to deal with the reference blocks, however there are a number of factors and there is no universal rules. This work will discuss the problems and the vagueness of the criteria for repeated use of reference blocks for checking the PT system and investigate the comparability of the results of the repeated use due to the influence factors (time of penetration and development, surface temperature of reference block, indications observing conditins and different methods of cleaning the surface of blocks before testing). In the experimental part there are selected reference blocks subjected to various test conditions in different ranges of measurement values and were read the sharpness and contrast of indications and photographic were recorded. The results are compared in order to come to the conclusion how relevant and repeated use of reference blocks, and to provide us with credible information with regard to the impact of the studied factors. The above tests were conducted in accordance with the family of norms EN ISO 3452 [1] [2] [3] [4] [5].