Saccharide binding has been observed with demetallized concanavalin A in the presence of Ca(2+) only, using the fluorescent sugar 4-methylumbelliferyl alpha-D-mannopyranoside. At pH 7.2 both the nicked and intact forms of concanavalin A bound 4-methylumbelliferyl alpha-D-mannopyranoside with similar affinities. Competitive binding with methyl alpha-D-mannopyranoside was demonstrated. The association constants at 5 degrees C were 9.6 +/- 0.6 X 10(4) M(-1) for 4-methylumbelliferyl alpha-D-mannopyranoside and 1.1 +/- 0.3 X 10(4) M(-1) for methyl alpha-D-mannopyranoside. 4-Methylumbelliferyl alpha-D-mannopyranoside binding was also observed if demetallized concanavalin A was remetallized with less than stoichiometric amounts of Ca(2+). The association constants with low Ca(2+) concentrations were similar to those determined with saturating Ca(2+). With less than stoichiometric levels of Ca(2+), the number of sugar molecules bound per protein subunit was a reflection of the fraction of activated lectin subunits. These results show that saccharide binding activity of concanavalin A does not require a transition metal ion at pH 7.2; only Ca(2+) is required. At pH values near 5, where most previous studies have been carried out, both a transition metal ion and Ca(2+) are necessary.