OBJECTIVE The use of infusion filters in pediatrics is controversially discussed. Their application is an excellent opportunity to prevent complications, but there are researchers, who do not see any advantage in using inline filters. This paper describes the interaction of five different drugs with a positively charged as opposed to an uncharged polyethersulfone (PES(+) versus PES(0)) membrane. METHODS To measure the extent and the mechanism of interaction, PES(+) versus PES(0) membranes and furosemide sodium, potassium canrenoate, digitoxin, digoxin and adenosine, each 30μmol/l, were investigated. Salt ions with different hydrodynamic radii and different concentrations have been used in the eluents. RESULTS During furosemide sodium and potassium canrenoate filtration with PES(+), the onset of UV absorption depends on the electrolyte concentration in the eluent: the lower the electrolyte concentration the later the onset of UV absorption. A correlation between the hydrodynamic volume of the different salt ions used and the onset of UV absorption could be proven for both substances: The larger the hydrodynamic volume of the extrinsic ion, the later the onset of the UV absorption if the same electrolyte concentration was used. Due to a higher structural density of PES(+) than PES(0) a delayed onset of UV absorption during filtration of digitoxin and digoxin with the PES(+) membrane could be observed. No correlation between the hydrodynamic volume of the different salt ions used and the onset of UV absorption could be seen. With adenosine neither the filter type nor the electrolyte concentration or the hydrodynamic volume of the salt ions had an influence on the onset time of absorption. CONCLUSIONS The results obtained with the anionic drugs investigated are particularly relevant if low drug concentrations in a saltless infusion solution are applied in combination with a charged filter membrane. Therefore, for each infusion formulation, a careful selection of the filter material is essential.