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Crystal violet has an antibacterial action against Escherichia coli, Staphylococcus aureus, Streptococcus faecalis and Bacillus subtilis. The effect of the dye, measured as minimum inhibitory concentration or retardation of growth, increases as the pH rises from 6 to 8. Of the four species E. coli is most resistant to the dye; the resistances of the other organisms are similar. The mode of action put forward by Steam & Stearn (1928) that the action of crystal violet is due to the formation of an unionized complex of bacteria with dye, is supported. Gram-negative organisms, such as E. coli, have high isoelectric points and contain less acidic components than Gram-positive bacteria which usually have lower isoelectric points, so the former combine with crystal violet less readily and are more resistant to the dye. In extension of this theory, the negative charge on bacteria is increased as the pH of the medium is increased, and the organisms become more sensitive to dye. Evidence is presented which refutes the theory of a poising of the redox potential by crystal violet suggested by Dubos (1929) and Ingraham (1933).