Escherichia coli cells were grown in the presence of L-serine and gallium(III) nitrate a different molar ratios. Under these conditions ultrastructural changes were observed in the cells when examined under the electron microscope. Although some changes were seen inside the cell the major modifications were observed at the cell surface. These changes appeared to involve both the cell and the peptidoglycan layer. Autoradiography at the electron microscope level undertaken with similar mixtures and containing L-(3 - 3H) serine showed silver grains at or near the cell surface. In some cases, surface modifications were so pronounced that they resulted in the E. coli appearing as sheets of cells. No cell surface changes were detected when mixtures of L-serine and potassium tetrachloropalladate(II) were used as modifying agents. With the palladium(II) mixtures all changes observed were intracellular. These modifications included the appearance of membrane-bound vehicles, clumping of the cytoplasm and changes in the nucleoplasm. Autoradiography carried out in the presence of L-(3 - 3H) serine showed a significant proportion of silver grains over the nuclear region. A pure palladium(II) complex of L-serine was examined as a modifying agent in the concentration range 1-9 μ/cm3 resulting in very pronounced modification of the cells when exposed to higher concentrations.