A concanavalin A (Con A)-binding polypeptide with a molecular mass of 24 kD (termed "SRgp24") was associated with the intercellular space of Mesembryanthemum crystallinum L. callus. When callus was grown in medium containing between 0 and 100 mM NaCl, SRgp24 was detected by Con A binding. Increasing the NaCl concentration to 200 mM caused a reduction in the amount of SRgp24 within 3 d, and returning the callus to medium without salt resulted in an accumulation of SRgp24. Immunoblot analysis showed that appreciable amounts of SRgp24 accumulated in the leaves when plants were grown under sodium-limiting conditions. Unlike most of the cell-wall Con A-binding proteins in M. crystallinum callus, the carbohydrate moiety of SRgp24 was resistant to endoglycosidase H digestion. After purification of SRgp24, the N terminus was sequenced and found to share 55 to 60% identity with the N terminus of osmotin, a group 5 pathogenesis-related protein (PR-5) that accumulates in salt-adapted tobacco cell suspension. Immunocytochemical assays, with affinity-purified antibodies to SRgp24, indicated that SRgp24 preferentially accumulated in the cell-wall region. We conclude that SRgp24 is a salt-responsive glycoprotein related to the PR-5 family in M. crystallinum.