The cellulosome multienzyme complex was dissociated into 12-14 components when incubated at 30 degrees C in a reaction mixture that was buffered at pH 5.0 and was 50 mM with respect to sodium dodecyl sulphate and 10 mM with respect to both ethylenediaminetetraacetic acid (EDTA) and dithiothreitol (DTT). The dissociated components reassociated into a complex when dialysed against 20 mM TRIS/HCl buffer, pH 7.7, containing 2.5 mM DTT. When incubated in the presence of Ca2+ and DTT the reassociated complex had the same activity to hydrogen-bond-ordered cellulose as the undissociated cellulosome. However, when Ca2+ ions were incorporated into the TRIS/HCl-DTT dialysis medium the reconstituted complex had very little activity towards cellulose. Other divalent cations such as Mg2+ and Ba2+ had the same effect, but the monovalent cation Na+ resulted in a complex that was very active on crystalline cellulose. The results are interpreted as indicating that the divalent cations bind to one or more of the dissociated polypeptide components and induce changes in conformation that prevent their reassociation into a complex with activity towards crystalline cellulose.