The antioxidative activity of some newly synthesized aminomethanephosphonic acid derivatives was studied. The compounds studied differed in their polarity and the hydrophobicity of the electronic substituents at their nitrogen and phosphorus atoms. It was found that all the aminophosphonates studied, both cyclic and acyclic, protected erythrocyte membranes against peroxidation to some extent. The effect was somewhat weaker in the case of cyclic compounds, and for erythrocytes irradiated with UV light. The cyclic compounds provided no protection of erythrocytes illuminated by natural light. The observed differences between the antioxidative activities of cyclic and acyclic compounds are probably related to differences in their ability to incorporate into the lipid phase of erythrocyte membranes. Once incorporated, they change the fluidity of the membranes. The extent of those changes was determined in fluorescence measurements. Generally, they were found to be more pronounced in the case of acyclic aminophosphonates, although as regards other structural differences between particular aminophosphonates, a clear picture of the relationship between structure and effect is more difficult to obtain. No correlation was found between the antioxidative efficiency of the compounds and the fluidity changes they induce.