Development of a highly sensitive silver stain permits the characterization of trace cellular and body fluid proteins separated by the two-dimensional electrophoresis technique of O'Farrell. Many of the proteins detected by the silver stain in urine, spinal fluid, amniotic fluid, and cells were undetected with the widely used Coomassie blue stain. Trace polypeptides observed in Escherichia coli cell lysates with this silver stain could be detected previously only by growth in radioactive precursors followed by lengthy autoradiography. In situations that do not permit the use of radioactive labeling, as in human clinical studies, the enhanced ability to detect proteins achieved by the silver stain will facilitate metabolic studies and the screening for protein abnormalities in mutational studies and in genetic diseases.