Summary Using 68 Ge and 71 Ge as tracers in silicate metabolism of the marine centric diatom Cyclotella cryptica , cell free extracts of labelled cells (supernatant 38,000 Xg) were analysed by thin layer chromatography. Only after hydrolysis at 100°C, a soluble fraction was found. In in vitro studies, a small fraction of the label could be bound to components of the cell free extract. Instead to components of cell free extracts, binding of 68 Ge(OH) 4 was possible also to the polyamino acid polyserine, but not to polymethionine. Several other low molecular weight substances with hydroxyl groups, such as mannitol, ascorbic acid, boric acid and isocitric acid were also used for binding studies without significant effects. The results are discussed in relation to the finding of a high serine and threonine content in the silicalemma of diatoms by other authors.