The experiment was conducted on lambs of the Polish Lowland sheep (PLS) and a synthetic BCP line. The redox potential was assessed in blood of lambs with different genotype and sex, which were fed feed mixtures with various contents of a proteinxanthophyll (PX) concentrate from alfalfa (1.5% and 3%). The 1.5% addition of PX concentrate to feed stimulated the response of the antioxidant defence mechanisms of the lambs to a greater extent than the 3% addition. The study showed that sex of the lambs had no significant effect on changes in the redox parameters of blood. The PLS genotype of lambs determined higher values of superoxide dismutase (SOD), total antioxidant potential of plasma (FRAP), malondialdehyde, and vitamin C than the BCP genotype. Along with the growth of the lambs, a descending tendency was observed in SOD and catalase levels, and an ascending tendency in FRAP value.