During the isolation of the amatoxin RNA-polymerase B-complex from calf thymus tissue we also isolated a protein (ABP) which shows such strong affinity to [3H)amanin that significant binding occurs at low concentrations (10-7M) of the label. The presence of a new amatoxin-complex is demonstrated by coprecipitation of amatoxin and ABP with ammonium sulphate and the common chromatography on phosphocellulose and Sephadex G-25. The new protein ABP is characterized by denaturating sodium dodecylsulphate-gel electrophoresis. The molecular masses of both main bands - possibly subunits of ABP - are determined as 100000 and 10000 - 15000 Dalton and different from the subunit pattern of RNA-polymerases B and C.