Apoptosis, which occurs in highly proliferating tumours spontaneously or during anticancer therapy, many lead to an elevated concentration of circulating nucleosomes in blood. In order to quantify the concentration of nucleosomes, we used the Cell Death Detectionplus-ELISA (CDDE) (Boehringer Mannheim, Germany) based on antibodies against histone and DNA, adapted it to the demands of liquid materials and standardized test performance and handling of serum. Furthermore, we investigated serum samples of 185 patients with solid tumours (additionally 24, treated with radio- or chemotherapy), 30 with acute inflammations and 50 healthy persons. Many patients with tumours (78%) and inflammations (77%) showed higher concentrations of serum-nucleosomes (> 100 AU), whereas 96% of all healthy persons had low values (< 100 AU). Follow up-studies revealed an early peak after initiation of therapy and correlated to the clinical outcome. The concentration of nucleosomes is a sensitive marker of cell death and could be used for monitoring the efficacy of antitumour therapy.