Balb/c Mice were immunized with splenic isoferritin. Spleen cells from immunized Mice were fuzed with SP2O myeloma cells. Four monoclonal antibodies designated respectively M29, M211, M386 and B8 were selected. Various isoferritins were analysed by immunodiffusion (Ouchterlony technic). With these monoclonal antibodies and with Rabbit polyclonal sera: human basic and acidic isoferritins and Horse spleen ferritin. Ferritin could be precipitated by these monoclonal antibodies. These results confirm that the ferritin molecule consists of repeating antigenic sites. No immunoreactivity difference could be detected between acidic and basic human ferritine. Species specificity was recognized. The high affinity constant of the M29 monoclonal antibody allowed development of a standardized radioimmunossay of ferritin.