We have studied the electrophsiological effects of IgG obtained from four patients with Lambert-Eaton myasthenic syndrome (LEMS) (two with small cell carcinoma), using the mouse passive transfer model. Mice received LEMS or control IgG or plasma, 10 to 60 mg daily. Microeletrode intracellular recordings were from diaphragm muscle. LEMS IgG and plasma decreased end-plate potential quantal content similarly, confirming IgG as the active factor. LEMS IgG was equally effective C5-deficient mice, indicating that late complement components are not required. The time course of decline and recovery of quantal content closely followed that of the human IgG in the mouse serum, with time to half-maximal effect of about 1.5 days in each case. Binding/dissociation of IgG or down/up regulation of the antigenic determinants, possibly Ca2+ channels, has a half-life of between 2 and 36 hours. The results confirm our concepts that IgG antibody to nerve terminal determinants underlies the disorder of transmitter release in LEMS.