Lake Baikal harbors the largest diversity of sponge species [phylum Porifera] among all freshwater biotopes. The abundantly occurring species Lubomirskia baicalensis was used to study the seasonal silicatein metabolism; the spicules of this species have an unusually thick axial filament, consisting of silicatein, which remains constant in diameter during their growth. In the course of maturation, the size of the silicic acid shell grows, until the final diameter of the spicules of about 8 μm is reached. The seasonal content of silicatein was assessed by use of antibodies raised against silicatein; they stained specifically the axial filaments. In addition we determined, by application of the enzyme-linked immunosorbent assay system, that the proteinaceous content of the spicules, the silicatein, increases from spring to late summer by 8-fold. As molecular markers to quantify the seasonal changes in expression levels of genes coding for proteins/enzymes, the genes for the calumenin-like protein and the kinesin-related protein, were selected. The expression of calumenin-like gene, involved in the intracellular signaling, is highest during September, whereas the expression of the kinesin-related protein does not change during the annual course. These results suggest that the highest metabolic activity of L. baicalensis occurs in late summer (September), in parallel with the highest accumulation of silicatein, a structural protein/enzyme of the spicules.