1. Introduction Gonflicting data on ferritin substructure have been produced after the report of ferritin charge heteroge- neity [ 1,2]. In horse spleen ferritin only one subunit type of 18 000-19 000 mol. wt was found by a number of analyses [3]. In the same type of ferritin a consistent amount of 11 000 and 8 000 mol. wt ‘subunits’ were also found [4]. Moreover 19 000 and 14 000 mol. wt ‘subunits’ were found in rat ferritins [5]. We showed that human liver ferritin run in reducing conditions on acid-urea gel electrophoresis displayed only one major band, while heart ferritin displayed two: one in common with the liver and another of slower mobility [6]. The ratio between these two bands, in these and other ferritins, was well correlated with their isoelectric points and immuno- logical reactivities [7]. After improving the resolution of sodium dodecylsulphate (SDS)--gel electrophoresis by performing it in a gradient of polyacrylamide, we found that two major bands with similar mobility were present in all the ferritins analyzed, one band was called H subunit and of mol. wt -21 000, the other L, of 19 000 [8]. Interfacing acid-urea- with SDS-electrophoresis it was found that the two systems separated the same peptides, and a good correlation was evident between H:L ratio and the isoelectric points of isoferritins from various species [8,9]. Other evidence suggested that the smaller peptides may arise from proteolytic digestion of the 19 000 and 21 000 mol. wt subunits [8,10]. Another form of subunit heterogeneity has been suggested from isoelectric focusing analyses in urea gels, in which up to 7 bands have been observed [ 111; however it is not clear whether this heterogeneity is