Etherification of alpha-amanitin with tritiated methyl iodide yielded a radioactively labeled amatoxin of high specific activity (similar to or approximately 4 Ci/mmol) which, in its inhibition capacity for RNA polymerase II, was very similar to alpha-amanitin. The labeled toxin was used successfully in binding assays with RNA polymerases II and in radioimmunological determinations of amatoxins. If long-chained alkyl bromides were reacted with alpha-amanitin, lipophilic ether derivatives were obtained with a facilitated penetration capacity into cells. As a consequence of the improved permeability, two derivatives, O-hexyl- and O-decyl-alpha-amanitin, were more toxic in vivo than alpha-amanitin, although their affinity to RNA polymerases II was much reduce. By reaction of N-tert-butyloxy-carbonyl-N'-(6-bromocaproyl)ethylenediamine with alpha-amanitin, a ten-atom spacer with a terminal amino group could be introduced into the toxin, which allowed the attachment of alpha-amanitin to proteins, solid-phase supports, or reporter groups. For example, by reaction with fluoresceinyl isothiocyanate, a fluorescent amatoxin was prepared for visualizing amatoxin-binding structures in cells. After succinylation of the spacer moiety, alpha-amanitin could be attached to proteins, e.g., fetuin, yielding a derivative with good antigenic properties. When an alpha-amanitin derivative was coupled to Sepharose 6B, an adsorbent for affinity chromatography was obtained suitable for a one-step purification of amatoxin-binding immunoglobulins from the sera of immunized rabbits.