The DNA released into the culture medium after phytohemagglutinin (PHA) stimulation of human peripheral blood lymphocytes has been purified and characterized. It is double stranded, sediments at 7-8S in alkaline sucrose, and has a Tm determined optically and by thermal elution from hydroxyapatite that is substantially lower than that found for lymphocyte cell DNA. Media DNA contains a major component reassociating with an average Cot-1/2 of 87 mol X s/liter, compared to a Cot-1/2 of 770 mol X s/liter for the unique fraction of cell DNA as measured by reassociation in 0.6 M Na+. This component of media DNA consists of unique sequence elements which are largely shared in media DNA preparations from cultures derived from different cell donors. The marked difference between media DNA and cell DNA indicates that media DNA is not derived from cell death and lysis, rather than some unique portion of lymphocyte DNA is apparently excreted from the cells during PHA-stimulated growth.