Phenylmercuric borate (PHB) is very rapidly incorporated into the cells of Escherichia coli, and binds directly to its sites of fixation. The rapid incorporation of PHB probably results from the lipophilic character of the [phenylmercuric]+ cation. The distribution of PHB in the cell is fairly uniform if one compares with that of the proteins. This shows the low specificity of PHB with regard to proteins, all of which appear to be potential sites for fixation. However, from the structural point of view, PHB is more differentiated in its distribution. The cell wall of E. coli and the soluble cytoplasmic fraction do not constitute important sites of fixation. On the contrary, the cytoplasmic membrane appears to be a preferential site of fixation. Moreover, it is very probable that the distribution of PHB in vivo should be even more likely at the cytoplasmic membrane since the process of homogenization of the cells solubilizes a large part of the proteins associated to the membrane. On the membrane, an important part of PHB seems to be associated with the ribosomes and particularly to the ribosomal proteins.