An enzyme catalyzing the conversion of mu- to kappa-carrageenan has been demonstrated in both haploid and diploid plants of Chondrus crispus. It acts at the polymer level producing 3,6-anhydro-d-galactose with the stoichiometric release of sulfate. Two-thirds of the recoverable enzyme was associated with the 15,000g pellet most of which could be solubilized by passage through a Ribi Cell Fractionator. The enzyme precipitated between 2.65 and 4.24 m (NH(4))(2)SO(4) and was partly purified on DEAE-cellulose columns. This sulfohydrolase has a pH optimum near 6.5 and is inhibited by molybdate, phosphate, sulfate, tungstate, cysteine, ATP, GTP, UDP, and by lambda-carrageenan. No activator was found. The enzyme showed a similar affinity for several preparations of mu-carrageenan and for the kappa-carrageenase-resistant fraction from kappa-carrageenan thus confirming that the latter is a biosynthetically unfinished molecule.A comparable extract from Gigartina stellata gave a higher specific activity for the sulfohydrolase, but was otherwise quite similar to the Chondrus enzyme.