We investigated human blood erythrocytes under oxidative stress in vitro and established a correlation between composition and state of lipids and changes in erythrocytes structure under induced oxidative stress. These changes may serve as an indicator of not only the erythrocyte state but of systemic processes that occur at the level of the whole organism, including various pathologies as well. We found that a pyrimidine derivative xymedon used in the present study is an effective inhibitor of oxidative processes. Xymedon may be useful as an antioxidant for preserving the structural and functional characteristics of the erythrocytes in the treatment of organisms exposed to physical and toxic factors causing oxidative stress.