Ferritin is an iron-storage protein which is found in eucariotes as well as in procariotes [1]. In mammals ferritin is present in almost every tissue, at higher concentrations in iron-rich organs such as spleen and liver. Ferritins extracted from a single tissue families of different isoferritins which can be distinguished according to their surface charges and/or immunological properties [2]. The prototype ferritin molecule is an oligomer of 24 subunits, arranged in 432 symmetry, which form an inner cavity of approximately 75 Å diameter. This hollow oligomeric molecule readily accommodates an inorganic matrix of hydrated ferric oxide phosphate (up to 4500 iron atoms stored) which possesses crystalline order. Ferritin heterogeneity can be related at a molecular level to the different associations of two subunit types in the protein shell. In liver ferritins the proteic shell is composed mainly of the so called L subunits (18,500 mol. weight), while H-type subunits (21.000 mol. weight) are predominant in heart ferritins [3]. Most of what is presently known of the three-dimensional structure of ferritins comes from the crystallographic investigations on spleen ferritin [4], which readily crystallizes from cadmium sulfate solutions (Space Group F432; a = b = c = 184 Å, one subunit per asymmetric unit). Much less is known about the structure of the H subunit-rich isoferritins, dealt with in this communication. In the course of the last year we have achieved the crystallization of horse heart isoferritin (both holo and apo forms) under different physico-chemical conditions. In particular we were able to grow crystals from protein solutions containing MPD (2-methyl-2,4-pentane diol) and in the presence of polythylene glycol (av. mol. weight 4.000 ÷ 6.000), but not from CdSO 4 (Fig. 1 a,b). Thus heart ferritins cannot be crystallized under the same physicochemical conditions which are used for the crystallization of spleen ferritins. It is interesting to note that in the presence of MPD the reverse is also true. ▪ The dimensions of the heart ferritin crystals obtained so far are too slight to allow any crystallographic investigation. All the crystals obtained are isotropic under the polarizing microscope, and thus belong to a cubic space group. This observation is in keeping with the molecular symmetry of the ferritin oligomer, which, in the case of the spleen protein, is coincident with crystallographic symmetry. Details of the crystalline forms isolated as well as of their growth solutions will be presented.