Abstract: THE characterisation of glutamate receptors in nerve and muscle membranes requires compounds which interact specifically with these receptors. In the vertebrate central nervous system various substances are reported to act as agonists or antagonists of glutamate (reviewed in refs 1 and 2). At the locust and crustacean nerve-muscle junctions, where the receptors are readily accessible for biochemical and pharmacological characterisation, comparatively few compounds activate the glutamate-sensitive receptors or antagonise the excitatory action of glutamate3–9 and so such substances are needed. We report here that the glutamate analogue, DL-2-ammo-4-phosphonobuytyric acid (DL-APB) (Fig. 1), inhibits the binding of glutamate to ‘receptor-like’ hydrophobic proteolipids isolated from locust muscle10,11 and antagonises the excitatory action of glutamate applied iontophoretically to receptors present in the locust muscle membrane. The shape and charge distribution of DL-APB are appropriate for interaction with glutamate receptors2,7,12.