The iron storage protein ferritin is the principal yolk protein in oocytes of the snails Planorbarius corneus L. and Lymnaea stagnalis L. This report gives an account of the isolation procedure and of some properties of snail ferritins. The isolation procedure includes a heat-denaturation step, gel filtration on Sepharose 6B and two ultracentrifugation steps followed by electrophoresis. Ferritins from both snails are highly reminiscent of vertebrate and plant ferritins in terms of heat stability, absorption spectrum and ultrastructure, and both share common antigen determinants with horse spleen ferritin. In different electrophoresis systems snail ferritins display considerable heterogeneity and microheterogeneity. Electrophoresis in the presence of sodium dodecyl sulphate (SDS) yields two major polypeptides with molecular weights of 19 000 and 24 000, which are interpreted to be authentic subunits of the ferritin molecule. Different organs and tissues of the snails differ in subunit composition. Midgut gland ferritin consists predominantly of the 19 000 Mr polypeptide, while in embryos only the 24 000 Mr band was found. No carbohydrates or lipids could be detected by staining acrylamide gels. Results from SDS/acrylamide electrophoresis, electrophoresis under non-denaturing conditions on gradient gels and from isoelectric focusing indicate that the ferritins of both snails are composed of at least two different types of ferritin that are tissue-specific. One ferritin is typical of somatic tissue (midgut gland) and is most probably a homopolymer of the 19 000 Mr subunit. The other ferritin is typical of oocytes, but since it is an exogenous protein it is also encountered in the midgut gland (the presumed site of yolk synthesis) and the haemolymph. Vitellogenic ferritin is either a homopolymer of the 24 000 Mr subunit or is predominantly composed of it. So far, there is no evidence for a precursor-product relationship between the two subunits.