Nascent synthesis and accumulation of germin and its mRNA mark the onset of renewed growth when wheat embryos are germinated in water. Germin is a water-soluble, pepsin-resistant protein that is not found in immature embryos, or in mature embryos before their germination. An antiserum was raised by injecting rabbits with germin that was freed of other proteins by pepsinization and gel filtration. The antiserum has been used to detect, in extracts of mature embryos from dry, ungerminated wheat grains, a protein that is antigenically related to germin. The antigenically related protein has bccn named pseudogermin. Pseudogermin accumulates, maximally, between 20 -25-days postanthesis, then declines appreciably in amount by 30-days postanthesis, in soluble extracts of immature embryos from several wheat varieties. The antiserum was also used to identify germin and pseudogermin among the proteins extracted from cell walls and to bind immunogold to cell walls preparatory to visualizing freeze-cleaved embryos by scanning electron microscopy. Wall-associated germin accounts for about 40% of the total germin in germinating wheat embryos. Appearance of germin in the apoplast is the most conspicuous germination-related change in the distribution of cellwall proteins. It seems that germin may act at the level of the apoplast and that pseudogermin may subsume the role of germin at low water potcntials during cmbryogenesis. The N-terminal eicosapeptide sequences in germin and pseudogermin are very similar but SDSjPAGE analysis detects discrete differences between the mobilities of their constituent monomers as well as gross differences between the stabilities of the parent oligomers. Like germin, pseudogermin is a water-soluble, pepsinresistant protein, but pseudogermin has unprecedented disulphide-independent thermostability propertics that have never been previously reported for a water-soluble oligomeric protein. Polysaccharides that co-purify with otherwise pure specimens of germin (and pseudogermin) have been isolated for analysis and shown to be highly substituted glucuronogalactoarabinoxylans. The possible biological significance of selective and tenacious association between germin and glucuronogalactoarabinoxylans is discussed in relation to cell expansion during embryogenic and germinative development of wheat, as are some peculiarities of amino-acid sequence that suggest a possible relation between germin and a proton-specific ion pump : gastric ATPase.If mature embryos ( z 5% water) from Ungerminated wheat grains [I] Ahhrrviation. NaCI/P,, phosphate-buffered salinc.5 -10-h postimbibition, coincident with the onset of a secondary uptake of water, there is nascent synthesis of mRNA for a protein [3 -61 that we have called germin [7, 81 (for review see [9]).Accumulation of germin and its mRNA are conspicuously associatcd with the secondary water uptake that raises the water content of the embryos rrom about 60% at 5-h postimbibition, to about 85% by 24-h postimbibition. Structures for germin (monomer M , z 25000; ...