source (besides cultivation in an ideal pH), temperature and incubation time. Aspergillus sp. GM4 tannase pro- duction was increased with low MgSO4 supplementation. Many enzymes require metal ion activators to achieve full catalytic activity. At low concentrations, metal ions act as enzymatic cofactors, thereby increasing the cata- lytic activity of the enzyme, whereas at high concentra- tions, metal ions can reduce catalytic activity [25]. Mg2+ is a well-known enzyme cofactor. The supplementation of the culture medium with salts containing magnesium is important for adequate action of magnesium-depend- ent enzymes, which are present in the general metabolic pathways and in synthesis of nucleic acids.