The deoxyribonucleic acid (DNA) from cells ofEscherichia coliB, after rigorous extraction, was found to contain residual amounts of protein, the quantity of which was determined by the medium in which the cells were grown. The DNA of cells grown in a minimal salts medium contained from 10 to 15 times more protein than the DNA of cells grown in an enriched medium. The addition of amino acids, vitamins, or nucleic acid bases to the growth medium resulted in a decrease in the amount of residual protein and an increase in the sensitivity of the cells to ultraviolet light. Of the enrichments tested, the amino acids produced the greatest decrease in the quantity of residual proteins and the largest increase in sensitivity to ultraviolet. The type of mutant cell produced by ultraviolet irradiation was found to be strongly influenced by the ingredients of the growth medium. Few mutants were found after the irradiation of cells grown in the minimal salts medium but when the growth medium was enriched with amino acids, many mutants requiring amino acids appeared. Similarly the addition of vitamins or bases resulted in the production of vitamin-and base-requiring mutants. It is suggested that these residual proteins become attached to specific sites on the DNA during the operation of certain genes and this results in an increase in the ability of the DNA to withstand the damaging action of desiccation and ultraviolet light.