Background and So far many protein factors capable of inhibiting the immune system are secreted by different cancer cell lines in the culture supernatant and characterized. Ïn this report, the release of DNÂ fragments into the culture supernatant of human promyelocytic cell line, HL-60, with immunosuppressive activity was investigated. HL-60 culture supernatant was concentrated and applied to ion-exchange DËÂË-Sepharose ÇL-6B column. The column was eluted with NaÇl gradient and the fractions were collected for assay. Fractions with immunosuppressive activity were purified with HPLÇ. The data show that trypsin and RNase have no effect on antiproliferation, but DNase destroyed the immunosuppressive activity. The DNÂ with irreversible immuno-suppressive activity was found to be double-stranded with the molecular weight range between 0.6 and 1.5 kilobases. This DNÂ did not change the cell number and viability and as a result was not cytotoxic. Çell cycle analysis has suggested that DNÂ molecule could arrest the mitogen-stimulated lymphocytes in G1 phase, and prevent the cells from entering into the proliferating S phase. Çonclusion: These data show that DNÂ like immunosuppressive proteins could inhibit the proliferation of lymphocytes as well as the proliferation of some cancer cells.