The linked optical test for the analysis of digital-like substances with the enzyme Na+/K(+)-ATPase was investigated with regard to inhibition by components of the reaction medium. Most of the tested inorganic salts influenced the activity of the enzyme. However, the concentrations of the salts in human tissues and fluids are too small to cause measurable effects. Higher concentration of salts, which may be obtained by the preparative treatment of clinical material can influence the test. The determination of a reference value is recommended in these cases. The organic solvents DMSO and Methanol influenced the activity of the Na+/K(+)-ATPase, too. While the influence of DMSO in concentrations below 50% (v/v) was negligible, the measurement of a reference sample at higher DMSO concentrations and for methanolic samples is necessary.