This study aimed to identify and to evaluate the level of potency fluctuations of the pertussis component of Polish-produced DTP vaccine lots produced within 1972-2001 due to the changes having occurred in production and potency testing procedures. The study confirms that higher potency values were obtained for vaccine lots produced since 80-ties, e.g. after changes of: references lots (1975), vaccine strains (1978) and source of animals used in Kendrick tests (1979). Additionally, the comparisons performed revealed a down trend in potency levels within 1992-1999 correlating to the lowering of the number of IOU/dose.