When human lymphocytes are cultured in the presence of phytomitogens, 70-90% of the cells undergo blast transformation and synthesize DNA. However, less than 40% of these lymphocytes actually undergo mitosis while 35-90% of the newly synthesized DNA is excreted into the media. The release of DNA by the cells is selective since experiments with [(14)C]uridine indicate that RNA is not lost into the culture media. DNA excretion occurs under many culture conditions. The excreted DNA has an estimated molecular weight of 3 to 12 x 10(6) as determined by gel filtration on Sepharose 2B. It forms a single sharp peak at a density of 1.055 g/cm(3) when examined by sucrose density gradient centrifugation, suggesting that the DNA is complexed to protein or lipid.