We present a new method for visualizing proteins electrophoresed in sodium dodecyl sulfate-polyacrylamide gels. After electrophoresis, gels are incubated in CuCl2 to produce a negative image of colorless protein bands against a semiopaque background. Gels are stained completely within 5 min, do not require destaining, and can be stored indefinitely without loss of the image. Because proteins are not permanently fixed within the gel, they can be quantitatively eluted after chelation of Cu with EDTA. The sensitivity of the CuCl2 stain falls between that of Coomassie blue and silver. We anticipate that CuCl2 will be useful in the rapid analysis of proteins by polyacrylamide gel electrophoresis and in the preparation of purified polypeptides by elution from gel slices.