The effects of circulating anticardiolipin (ACL) antisera in lupus patients on the LP5 central neuron of snail were studied. Both GABA and glutamate increased a chloride conductance of the LP5 neuron. The ACL antisera decreased the GABA-elicited responses in a concentration dependent manner while it had no effect on glutamate-elicited responses. The ACL antisera affected neither the resting membrane current, nor the membrane conductivity of neuron. Antisera without the activity of anticardiolipin did not decrease the GABA-elicited responses. The seizure incidence of the patients with higher ACL antisera levels is also higher. It is concluded that ACL antisera inhibited the GABA ionophore receptor complex in a snail central neuron.