High recoveries of tryptophan and cysteine were achieved by 12.5 min of hydrolysis with mercaptoethanesulfonic acid vapor. Proteins (1-100 micrograms) were modified by vapor-phase S-pyridylethylation before hydrolysis. The modified proteins were hydrolyzed with the vapor of 2.5 M mercaptoethanesulfonic acid at 176 degrees C. This method promoted efficient hydrolysis of the peptide bonds in proteins and resulted in high recoveries of both tryptophan and cysteine, of 90% or greater, in addition to the other amino acids.