Shiga toxin has a single A subunit non-covalently associated with a pentamer of B subunits. The toxin has a trypsin-sensitive region near the COOH-terminal end of the A-chain, and upon cleavage, two disulfide bonded fragments, A1 and A2, are generated. These fragments are also formed upon incubation with cells. The disulfide loop contains the sequence (Arg-X-X-Arg), which is a consensus motif for cleavage by the membrane-anchored protease furin. We found that a soluble form of furin cleaves intact A-chain producing A1 and A2 fragments, and furin also seems to be responsible for rapid cellular cleavage of Shiga toxin. LoVo cells, which normally do not produce functional furin, cleave intact A-chain very efficiently when transfected with furin (LoVo/fur), whereas a control cell (LoVo/neo) cleaves the toxin very slowly. To investigate the role of this cleavage for intoxication of cells, we studied the ability of unnicked and furin-nicked toxin to inhibit protein synthesis in LoVo/fur and LoVo/neo cells. LoVo/fur cells were intoxicated equally well with unnicked and nicked toxin, whereas in LoVo/neo cells nicked toxin was about 20 times more active than unnicked toxin. The results suggest that cleavage of Shiga toxin is important for intoxication of cells, and they indicate that furin can cleave and thereby activate Shiga toxin in cells.