Extracts of leaves of Ginkgo biloba (family Ginkgoaceae), widely used in the treatment of peripheral and cerebral circulatory disorders as well as for dementia of different aetiology, contain long chain alkylphenols (with allergenic, cytotoxic, mutagenic and tumour-promoting properties) together with the extremely potent allergens, the urushiols. Such hazardous compounds can be present only in very low concentrations in phytopharmaceutical preparations and hence, for the purposes of drug safety, techniques must be available for the identification and quantification of these allergens at extremely low levels in refined manufactured materials. GC-MS analysis of samples collected at various stages during the production process of a standardised extract of G. biloba (EGb 761) demonstrated that all alkylphenols present in the primary acetone extracts were removed in parallel with the same efficiency irrespective of their aromatic substitution pattern. Furthermore, in the final product the content of urushiols was generally below the detection limit of 0.03 ppm. Therefore, it is concluded that demonstrating the absence of the predominant, and easily quantifiable, ginkgolic acids provides a reliable means for the control of pharmaceutical quality of the final product.