The mercuric bromophenol blue (Hg-BPB) staining technique has been developed for the cytological demonstration and measurement of proteins. This method, derived from protein chemistry, has been adapted for use in cytology, offering a simple and effective way to stain and measure proteins in biological samples. The technique involves the use of a reagent composed of mercuric chloride and bromophenol blue, which binds to proteins and produces a visible stain. The staining procedure involves immersing the sample in the reagent for a specified time, followed by washing and rehydration to enhance the staining effect. The Hg-BPB method is particularly useful for distinguishing protein structures that are difficult to visualize with other staining techniques. It provides good contrast and clarity, making it suitable for the study of various cellular structures, including cilia, spindle elements, and lamp brush chromosomes. The method also allows for the photometric estimation of protein concentrations, as the staining follows the Beer and Lambert laws, with an absorption maximum at 610 millimicrons. The technique is specific for proteins and certain peptides, and it has been shown that basic proteins bind the dye even in the absence of mercury, while other proteins bind through coupling with mercury. The staining is enhanced when nucleic acids are removed, indicating that nucleic acids may interfere with dye-binding groups. The Hg-BPB method is applicable for the comparison of total protein concentrations, as the variation in dye-binding among different proteins is relatively small. It is also useful for the approximation of large differences in protein concentration. The method has been shown to be effective for the analysis of proteins and protein mixtures, with excellent correlation between the amount of protein present and the amount of dye bound. The Hg-BPB method offers a valuable complement to other selective staining techniques, such as the Millon method for phenolic residues and methods specific for SH groups. It provides a versatile and reliable approach for the study of protein distribution and concentration in cytological preparations.The mercuric bromophenol blue (Hg-BPB) staining technique has been developed for the cytological demonstration and measurement of proteins. This method, derived from protein chemistry, has been adapted for use in cytology, offering a simple and effective way to stain and measure proteins in biological samples. The technique involves the use of a reagent composed of mercuric chloride and bromophenol blue, which binds to proteins and produces a visible stain. The staining procedure involves immersing the sample in the reagent for a specified time, followed by washing and rehydration to enhance the staining effect. The Hg-BPB method is particularly useful for distinguishing protein structures that are difficult to visualize with other staining techniques. It provides good contrast and clarity, making it suitable for the study of various cellular structures, including cilia, spindle elements, and lamp brush chromosomes. The method also allows for the photometric estimation of protein concentrations, as the staining follows the Beer and Lambert laws, with an absorption maximum at 610 millimicrons. The technique is specific for proteins and certain peptides, and it has been shown that basic proteins bind the dye even in the absence of mercury, while other proteins bind through coupling with mercury. The staining is enhanced when nucleic acids are removed, indicating that nucleic acids may interfere with dye-binding groups. The Hg-BPB method is applicable for the comparison of total protein concentrations, as the variation in dye-binding among different proteins is relatively small. It is also useful for the approximation of large differences in protein concentration. The method has been shown to be effective for the analysis of proteins and protein mixtures, with excellent correlation between the amount of protein present and the amount of dye bound. The Hg-BPB method offers a valuable complement to other selective staining techniques, such as the Millon method for phenolic residues and methods specific for SH groups. It provides a versatile and reliable approach for the study of protein distribution and concentration in cytological preparations.