The examination was made in the experimental room where 36 laying hens were kept in battery system. The hens were divided into three groups (12 birds in each group) – one control group (C) and two experimental groups E-1 and E-2. The birds from the control group were fed with the all-mash prepared according to standard recipe; the content of calcium was 3.0 %. The feed for the experimental groups contained also 3 % of calcium but 0.5 % (E-1) and 1.0 % (E-2) of Ca was from CaO₂ (51.8% of Ca, 16.7% of active oxygen, pH-value in 1% solution – 12.5). The experiment lasted 56 days. Total calcium concentration in blood serum samples and chosen qualitative parameters of eggs were analysed. It was found that eggshells thickness from laying hens of group E-2 was higher comparing with eggshells from control group and from group E-1 (p≤0.01). The eggshells from both experimental groups were more resistant (breaking strength) than eggshells from group C, especially the samples collected after 2 and 4 weeks of experiment (p≤0.01 and p≤0.05). The eggs from groups E-1 and E-2 had higher weight but that fact was not statistically confirmed. Calcium peroxide used in our experiment did not cause any discolouration of eggshells and had no influence on the colour of egg yolk. Calcium concentrations in blood serum were high (4.43 to 6.74 mmol/L) but not exceeded the values reported by other authors.