A simple and rapid procedure for the isolation of bacterial deoxyribonucleic acid is described. The deoxyribonucleic acid preparations are pure enough to determine the base composition by ultraviolet spectroscopy. Currently the most reliable test for the sepa- ration of staphylococci and micrococci is either an analysis of their deoxyribonucleic acid (DNA) base composition or the determination of their cell wall components. Since it is rather laborious and time consuming to determine such properties, these procedures have to be simplified for routine use in the laboratory. With regard to base composition, the isola- tion of pure DNA is wearysome, and optimal conditions for cell lysis often have to be found empirically. In addition, a large amount of cells needs to be harvested to gain sufficient DNA. A simple procedure for the isolation of DNA is described. The resulting DNA is pure enough to allow a reasonably accurate determination of its base composition by the method of Ulitzur (17). Since the DNA base composition of micro- cocci (65 to 75 mol% guanine plus cytosine (G+C)) differs greatly from that of staphylo- cocci (30 to 40 mol% G+C), even an approxi- mate determination of this value is sufficient to distinguish between these two genera. To isolate the DNA, the cells of a 300-ml culture grown overnight are harvested and sus- pended in a small amount (1 ml) of saline- ethylenediaminetetraacetate buffer (0.15 NaC1, 0.1 M ethylenediaminetetraacetate, pH 8.0). Proteinase K (50 pg/ml) is added to eliminate nuclease activity. After being mixed with glass beads (diameter, 0.17 to 0.18 mm) until a vis- cous consistency is reached, the cells are ground in a cell mill (Vibrogen cell mill, Buhler, Tubingen) for 10 min. The cell constitu- ents are then separated from the glass beads by a wash with 10 ml of saline-ethylenediaminetet- raacetate buffer and suction through a coarse, sintered filter. A 5-ml volume of the filtrate is made 1 M with respect to NaCl by adding 0.3 g of NaC1. A 2-ml volume of a 4% cetyltrimethylammonium bromide (CTAB) solution in 1 M NaCl and 2.0 ml of isopropanol are also added. The mixture is shaken vigorously with 1 volume of chloro-