Due to a possibly intended change in the technical manufacturing process, ZK. 21.955 (Hexabutyldistannoxane, 'TBTO') might contain a fraction of up to 15 % Monoethylpentabutyldistannoxane ('MEBTO'), A reduction of this percentage by mixing with charges without content of ethyl groups would easily be practicable. Since the investigations of Stoner et al. it is well known that Triethyltin (TET) compounds induce an edema in the white matter of the brain. This effect could be demonstrated histologically e.g. after three intragastric doses of 1.0, 2.0 or 3.0 mg/kg TET Bromide in 15 % Ethanol within two weeks. The objective of this study was to investigate whether the given maximum fraction of MEBTO would alter the toxicological properties of ZX. 21.955 which have been characterized in previous studies,in particular whether substitution of one butyl group of the TBTO molecule by an ethyl group would implicate a potential to damage the white matter of the brain like TET compounds. Furthermore a protocol based on recommendations of the 'Bundesgesundheitsamt' was followed to study possible changes of the toxic effects on the lymphatic organs, which had been noted in the preceding studies with TBTO.