The pathway of protein folding is now being analyzed at the resolution of individual residues by kinetic measurements on suitably engineered mutants. The kinetic methods generally employed for studying folding are typically limited to the time range of > or = 1 ms because the folding of denatured proteins is usually initiated by mixing them with buffers that favor folding, and the dead time of rapid mixing experiments is about a millisecond. We now show that the study of protein folding may be extended to the microsecond time region by using temperature-jump measurements on the cold-unfolded state of a suitable protein. We are able to detect early events in the folding of mutants of barstar, the polypeptide inhibitor of barnase. A preliminary characterization of the fast phase from spectroscopic and phi-value analysis indicates that it is a transition between two relatively solvent-exposed states with little consolidation of structure.