l-Hyoscyamine in the roots of Atropu belladonna L. was assayed by capillary gas chromatography (CCC). When the extraction and analysis were evaluated using a radioimmunoassay (RIA) established for l-hyoscyamine, the two methods were found to be positively and significantly correlated. Important characteristics of CCC and RIA methods in tropane alkaloid analysis of plant material are discussed. A prominent cross-reaction for apoatropine, a constituent of A. belladonna roots and leaves, was found in this study. This cross-reaction has not been investigated in other studies of hyoscyamine RIA.