The placement of a tryptophan residue into a single Ig-binding-domain of protein L from Peptostreptococcus magnus has been used to examine the binding interactions between the binding domain and kappa light chains (kappa-chains). The fluorescence intensity of the mutant domain increases on the formation of a complex with kappa-chains. This has been used to determine the Kd of the complex under a range of conditions by using both pre-equilibrium and equilibrium methods. The Kd values determined for the complex with kappa-chains at a number of different pH values are very close to those obtained with the wild-type domain, indicating that the mutation has not substantially affected its binding properties. Examination of the reaction between the mutant domain and kappa-chains by stopped-flow fluorescence shows that complex formation takes place by two discrete, sequential processes. A fast bimolecular reaction, with a rate constant of 8.3x10(5) M-1. s-1 (at pH8.0 and 25 degrees C), is followed by a slow unimolecular process with a rate (1.45 s-1) that is independent of the concentration of the reactants. This suggests that a conformational change occurs after the initial encounter complex is formed. The dissociation of the complex at equilibrium occurs in a single process of rate 0.095 s-1 at pH8.0 and 25 degrees C. Stopped-flow CD studies show that a slow decrease in ellipticity at 275 nm occurs with a rate of 1.3 s-1 when wild-type protein binds to kappa-chains, suggesting that the conformational transition might involve a change in environment around one or more tyrosine residues.