The complete primary structure of a cytotoxic 5 kDa polypeptide, viscotoxin A1, isolated from Viscum album L., has been determined by combining classical Edman degradation methodology with advanced mass spectrometric procedures. The same integrated approach allowed correction of the sequence of viscotoxin A2 and definition of the pattern of the disulfide bridges. The arrangement of the cysteine pairing was determined as Cys3-Cys40, Cys4-Cys32 and Cys16-Cys26. The primary structure of viscotoxin A1 shares a high degree of similarity with the known viscotoxins and more generally with the plant alpha- and beta-thionins. The pattern of S-S bridges determined for viscotoxin A2 and A1 is similar to that inferred by X-ray and NMR analysis in crambin and related to that present in alpha-purothionin and beta-hordothionin, thus indicating a highly conserved organization of the S-S pairings within the entire family. This arrangement of S-S bridges describes a peculiar structural motif, indicated as 'concentric motif', which is suggested to stabilize a common structure occurring in various small proteins able to interact with cell membranes. The distribution of the new variant toxin in different mistletoe subspecies was investigated. Viscotoxin A1 is abundant in the seeds of the three European subspecies of V. album whereas it represents a minor component in the shoots.