Humoral response to influenza vaccination being variable in uremic patients and being negatively correlated to red blood cell magnesium (RBC Mg) in non-uremic subjects, RBC Mg as well as plasma concentration of Mg were measured simultaneously with the antibodies titers after 1 and 2 injections of influenza vaccine in 21 non-uremic subjects and 47 patients on chronic hemodialysis. The RBC Mg and plasma Mg (MgP) were significantly higher in the dialysed patients (74 +/- 12 mg/l; 25 +/- 8 mg/l) than in the non-uremic subjects (54 +/- 5 mg/l; 19.4 +/- 1.5 mg/l). Furthermore, in the uremics RBC Mg was correlated to PMg whereas such a correlation was absent in the non-uremic patients. The humoral response of the uremic patients is depressed and becomes comparable to that of the non-uremic subjects after 1 injection only after 2 injections. In the uremic patients, the depressed humoral response is associated with higher RBC Mg (greater than 70 mg/l). The humoral response to influenza vaccine is depressed in uremic patients who have either the HBs antigen or no response to the hepatitis B vaccine but no link has been found between this immune status against the hepatitis B and RBC Mg. In conclusion, the humoral response of the uremic patients to influenza vaccine is depressed so that a second injection is necessary to give them sufficient protection.