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Department of Zoology, University of California, Berkeley 4, California

1. The mercuric bromphenol blue reaction as used for development of protein spots on filter paper has been found to be applicable to the cytological staining of proteins.

2. The optimum procedure is identical in detail with that described by Kunkel and Tiselius for filter paper spots, except that a neutral aqueous solution is substituteed for ammonia vapor in the final color development.

3. The sharp and intense staining of protein permits good differentiation of structures often difficult to observe, such as cilia, spindle elements, regions of spindle fiber attachment to chromosomes and "lamp brush" chromosomes.

4. The procedure is specific for proteins and those peptides which are not removed in the washing procedure.

5. The preparations stained by this procedure follow the Beer and Lambert Laws in microspectrophotometric measurements. The absorption maximum is at 610 millimicrons.

6. Basic proteins bind the dye under the conditions of the method even when Hg is omitted. Other proteins bind the dye by coupling through Hg. As expected, structures containing basic proteins show enhanced staining after removal of the nucleic acid.

7. The number of groups in various proteins binding the dye in this procedure varies somewhat, but the average is about one dye-binding group per 10 amino acid residues.