Limbic encephalitis is an inflammatory autoimmune disorder of the CNS, in some cases associated with the development of antibodies against voltage gated potassium channels (VGKC). These antibodies are immunoprecipitated after binding of ¹²⁵ I-a-dendrotoxin labeled VGKC. In this study we have investigated the effects of serum from a patient with limbic encephalitis on a K_v1.2-transfected fibroblast cell line (L929). VGKC-antibody titres were 2,7 nM (healthy control values <0,1 nM). Culture of L929 cells in the presence of 10% patient serum caused a massive decay in cell-population within three days. During the first two hours after serum application, however, no signs of cell damage could be observed. 2% serum did not affect cell viability. The patch-clamp technique was used to investigate the effects of patient serum on whole-cell K_v1.2 currents. 10% serum reduced potassium currents to 55% of control values, measured within 1h after application. Three day treatment by 2% serum also markedly reduced potassium currents. Our findings provide direct evidence that serum from a patient with limbic encephalitis interferes with K_v1.2 K⁺ conductance and cell viability, most likely via the action of VGKC-antibodies. Further studies will address the acute and chronic mechanisms of VGKC antibody effects.