RADIoimmunoassayforthequantitativedeterminationofblockingautoantibodiesagainsttheacetylcholinereceptorinhumanserum.TheARAbBlockingRIAisusefulasanaidinthedifferentialdiagnosisofmyastheniagravis(MG).Myastheniagravis(MG)isanacquireddisorderofneuromusculartransmissionassociatedwithabnormalmuscleweaknessandfatigABIlitiesonexertion.Thismuscleweaknessincreaseswithmuscleactivity,eventuallyreachingastateofpseudoparalysis.Aftermusclerelaxationthemotilityisrestored.Musclesthatarerequiredforbodyposture,includingfacialandneckmusclesaswellasocularmusclesandmusclesneededforspeakingandswallowingaremostaffected.Clinicalsymptomsmayberestrictedtoonemusclegroup,inparticulartheeyemuscles(ocularMG)orbecomegeneralized(generalizedMG).Myastheniagraviscanoccurfromearlychildhoodtooldageanditsprevalenceisabout5per100,000.Inwomen,thediseaseusuallypresentsbetweentheagesof20to40,whereasinmentheageofpresentationtendstobebetween40and60years.TheincidenceofMGis40to60newcasespermillionpeopleperyear.AutoantibodiesdirectedagainstacetylcholinereceptorsalongthepostsynapticmembraneareresponsIBLeforthemuscleweaknessandfatigabilitiesinMG.ThreefunctionallydifferenttypesofantibodiesagainstacetylcholinereceptorsaredetectableinpatientswithMG:receptorbinding,blockingandmodulatingantibodies.Relatedtotheblockingantibodiesthereisasignificantcorrelationbetweenthedegreeofacetylcholinereceptorblockadeandgeneralizationofmuscleweakness.Therefore,measurementofblockingacetylcholinereceptorautoantibodiesisofadditionalvalueindifferentialdiagnosisofMG.