The determination of the inhibition of PAF (platelet-activating factor)-induced platelet aggregation has been proposed as a biological standardization method for commercially available Ginkgo biloba extracts by measuring the characteristic pharmacological effect of ginkgolides in vitro. The determination is specific for ginkgolides A, B, C, and J and is not influenced by other constituents present in Ginkgo biloba extracts. IC50 values of ginkgolide B can be used to standardize various Ginkgo extracts produced by special extraction methods with respect to equi-effective ginkgolide B contents. In order to compare values obtained by a chemical-analytical procedure with those obtained by the biological assay, the equi-effective total ginkgolide content of each Ginkgo extract had to be calculated. Accordingly, the concentrations of the individual ginkgolides in the various Ginkgo extracts were determined chromatographically by assaying ginkgolides as trime-thylsilyl derivatives. Their individual contributions towards the measured in vitro effects were derived from their respective IC50 values. The calculated equi-effective total ginkgolide contents of the Ginkgo extracts were in good agreement with those obtained by gas chromatography. The results demonstrate that, in addition to a chemical standardization, the biological standardization of Ginkgo extract preparations is also feasible.