Combined GLC-mass spectrometry revealed that an unidentified sparteine isomer was the major component of an alkaloid extract of the aboveground portions of Lupinus argenteus Pursh. var. stenophyllus (Rydb.) Davis (Leguminosae). After isolation, this alkaloid was characterized as the least common of the known sparteine isomers, (-)-alpha-isosparteine. A preliminary pharmacological study showed (-)-alpha-isosparteine to have a more rapid onset and a shorter duration of action when compared with (-)-sparteine on rat myocardium.