A mature mutant ribose binding protein (RBP) of Escherichia coli was obtained by site-directed mutagenesis, replacing Thr-3 in the N-domain of wild-type mature RBP (WT-mRBP) with a Trp residue (N-Trp-mRBP). The equilibrium unfolding properties and the refolding kinetics of this protein were monitored by fluorescence and circular dichroism (CD). The stability of N-Trp-mRBP appears to be the same as that of C-Trp-mRBP, another mutant obtained by relpacing Phe-187 with a Trp, and lower than that of WT-mRBP. The overall refolding rate of N-Trp-mRBP was much smaller than that of C-Trp-mRBP which ,in turn, is similar to that of WT-mRBP. For the case of WT-mRBP, the rate constant obtained by Tyr fluorescence was identical to the value obtained by CD. But with C-Trp- mRBP, the rate constant from CD was smaller than the value from the Trp- fluorescence and this difference in the rate constants was much greater with the N-Trp-mRBP.