The presence of free serum DNA and/or antibodies against nuclear antigens was studied in 65 patients in chronic hemodialysis. Most of patients showed increased levels of both free single-stranded DNA (ss DNA) (28 out of 29) and native (double-stranded) DNA (n DNA) (25 out of 28) at the beginning of each dialysis session (69 ± 26 vs. 119 ± 15, p < 0.05) suggesting a rapid in vivo degradation of the n DNA released. 15 out of 65 patients (23%) developed low anti-ss DNA antibody titers by Millipore filtration assay. 2 of them presented anti-n DNA antibodies simultaneously. Isolated anti-n DNA, anti-Sm and anti-RNP antibodies were systematically negative in all patients. No relationship was found between the presence of anti-ss DNA antibodies and the type of nephropathy, the duration of treatment and the method of dialysis. The fate of a renal allograft was studied in 12 patients after having measured serum-free DNA and anti-DNA antibodies. The 4 patients with good graft function after 2 years of transplantation had significantly higher circulating ss-DNA levels than the other 8 patients whose grafts were rejected.