Sodium fluorescein angiography is a widely used routine ophthalmological diagnostic procedure which enables the study of chorioretinal microcirculation and consists of the injection of sodium fluorescein into the systemic bloodstream.

The aim of the present study was to evaluate whether or not fluorescein interferes with erythrocyte properties during the angiographic procedure.

In a group of 37 patients, 26 with non‐insulin‐dependent diabetes mellitus (DM) with and without retinopathy, and 11 without diabetes mellitus (non‐DM) although affected by other ophthalmological diseases, all undergoing routine angiography, blood samples were drawn before (T $_0$ ) and 30 min (T $_{30}$ ) after fluorescein injection. The erythrocyte aggregation index (EAI), membrane lipid fluidity and erythrocyte acetylcholinesterase activity were determined in both groups.

After fluorescein injection there was no statistical change in EAI and erythrocyte membrane fluidity in either group. Erythrocyte acetylcholinesterase activity, a marker of membrane protein integrity, decreased significantly ( $p<0.01$ ) in the DM group.

Membrane lipid fluidity did not change with fluorescein injection, however, (i) in the DM group erythrocyte membranes became more rigid than in the non‐DM (DPH: $p<0.01$ ); (ii) EAI and membrane lipid fluidity became significantly correlated ( $r= 0.6263$ , $p<0.05$ ) in non‐DM patients at T $_{30}$ .

In conclusion, fluorescein administration for angiographic procedures seems to interact with erythrocyte membrane, namely, in diabetic patients, which may interfere with the blood flow in the microcirculation.