The research was conducted on 200 piglets of PIC hybrid pigs, where the piglets were divided into 4 groups of 50 piglets (25 male and 25 female). The aim of the research was to determine the influence of supplementation of different sources and forms of selenium (Se) in the diet of fattened piglets on production properties and development of piglets, their health status and antioxidant activity. For the purposes of determining the total and average daily gains during the research, control weighings of all piglets were carried out. For the purpose of pathoanatomical examination, one piglet from each group was sacrificed on day 0 and five piglets from each group on day 42 of the experiment. After sacrifice, samples of duodenum, jejunum, muscle, peripheral lymph node, bone marrow, heart, kidney, thyroid and lung were taken from each piglet for pathoanatomical examination of the sacrificed piglet. For the purposes of hematological tests, biochemical analyzes and determination of enzyme activity, blood samples were taken from the superior vena cava (vena cava cranialis and 5 ml for biochemical analyses. On the 42nd day of the experiment, piglet blood was taken for biochemical analysis. 2 ml was separated from each sample blood and centrifuged at 3000 × g for 10 minutes at 4 ◦C to obtain plasma for biochemical tests. A total of 14 biochemical parameters were determined and analyzed. The activity of the enzymes glutathione peroxidase (GPx) and superoxide dismutase (SOD) and the content selenium (Se), zinc (Zn) and iron (Fe) in blood, tissues and organs. A positive effect of supplementing rations with Se from biofortified feeds on the health status and survival of piglets, and on their immune response was determined. In the same group of piglets, are statistically significantly higher values of hemoglobin concentration Between the experimental group that received organic selenium and the group that received nano selenium in the meal, statistically highly significant differences were found in the value of platelets in the blood of piglets. A strong positive correlation was also found between body masses, then between hemoglobin and hematocrit values, between MCV and MCH and MCH and MCHC. Regarding the content of selenium (Se) in the observed tissues and organs, a statistically significantly higher proportion of Se was found in the MLD of the piglets of the second experimental group compared to all other tested groups. The same case was found in the MTF, where statistically significantly higher selenium values were found in the piglets that received Se from biofortified feed compared to the MTF of the other tested groups. In the heart, liver, thyroid and kidneys, a statistically significantly higher selenium content was found in the piglets of the second experimental group, which received selenium in their meals from biofortified feed compared to the piglets of the other groups included in the research.