Abstract A simple, rapid method for the isolation of DNA from mammalian cells and from pneumococci is described. It involves repeated extraction of the cells, suspended in buffered 1 M NaCl, with water-saturated phenol. The product obtained is remarkably free of contamination by polypeptides, polysaccharides and RNA. The sedimentation characteristics of DNA isolated by this procedure have been investigated in the analytical ultracentrifuge, and the possible significance of the results is discussed.