Tabtoxinine-beta-lactam, a hydrolytic product of tabtoxin produced by Pseudomonas syringae pv. tabaci, apparently inactivates pea seed glutamine synthetase. Inhibition of the enzyme's initial velocity is linear over a range of 0.5 to 5 millimolar tabtoxinine-beta-lactam in the presence of 10 millimolar glutamate. A method for the purification of glutamine synthetase from dried peas is presented which gives a 30% yield with a 2,000-fold increase in specific activity. A method for obtaining highly purified tabtoxinine-beta-lactam and tabtoxin in good yields is also presented. The authenticity and purity of tabtoxinine-beta-lactam and tabtoxin were verified by chromatography, biological activity, and (1)H and (13)C nuclear magnetic resonance spectroscopy.