Incubation of an isolated human serum proteoglycan fraction with a tannic acid or oak tannin fraction caused a dose-dependent decrease in the total glycan content and a special diminution of a major proteoglycan band with molecular weight 112 kDa in samples as analyzed by polyacrylamide electrophoresis and with carbohydrate staining. Tannic acid was more active than oak tannins while the spruce tannin fraction caused almost no effects. The concentration of plant tannin extracts is often expressed as tannic acid equivalents, which may overestimate their biological activity in this model. This cell-free model may act as a versatile tool for studies on macromolemular interactions of plant polyphenols.