A rapid mixing-photocrosslinking technique has been developed to investigate the kinetics of protein-nucleic acid interactions. With this technique, binding of nucleic acid to protein is first synchronized by rapid mixing in a stopped-flow apparatus. The intermediates formed at different stages of the binding process are then "frozen" by photocrosslinking with a 10-μs uv light pulse at various times after mixing. By analyzing structural changes of these intermediates as a function of time, one can obtain the information concerning the dynamic aspects of the interaction. This technique may also be applied to other macromolecular interactions in biological systems.