Water hardness is a very important parameter of water quality which is able to affect the toxicity of many chemicals to fish. The aim of the present study was to examine the effect of two biphenyl mixtures: Delor 103 and Aroclor 1254 on the embryonic development and hatching of the Prussian carp (Carassius gibelio Bloch) depending on water hardness. Polychlorinated biphenyls are lipophylic chemicals which are able to affect the reproductive function in many vertebrates. The samples of eggs were obtained from 5 sexually matured Prussian carp females and activated with common carp (Cyprinus carpio L.) sperm. In both experiments (with Delor and Aroclor) to explain the relation between this two tested factors activated eggs were divided into 4 groups: two control groups (hard and soft water) and two experimental groups include PCBs mixtures at the concentration of 100 ng∙mL-1 diluted in hard or soft water. The percentage of living eggs after the first day of incubation, hatching rate and percentage of deformed larvae (therein dead) were observed. After 24 h of incubation there was no significant difference in the percentage of live eggs between groups either in experiment with Aroclor 1254 or Delor 103. In the case of hatching rate tested PCBs mixtures had no effect in both variants of water harness. Delor 103 had no effect on the percentage of deformed larvae of Prussian carp in both hard and soft water. Only the observation of the number of deformed larvae showed higher teratogenic influence of Aroclor 1254 in hard water. Summing up, our results showed that Delor 103 is not harmful to the development of activated Prussian carp eggs in both hard and soft water, but in the case of Aroclor 1254 diluted in hard water increased teratogenic effect was observed.