The composition and kinetics of accumulation of extracellular DNA in cultures of primary human endotheliocytes, cervical adenocarcinoma, and mycoplasma-infected cervical adenocarcinoma cells were studied. The content of DNA bound to cell surface did not change during culturing. The concentration of extracellular DNA in culture medium increased during the lag phase and at the beginning of the exponential growth phase, which probably attests to active secretion of DNA by cells. Spontaneous extracellular DNA synthesis was observed only in cell culture infected with mycoplasma.