The autosomal defect in Fas antigen leads CD4-CD8-T-cells to accumulate in lymph nodes and spleen of MRL-lpr/lpr mice. MRL-lpr/lpr mice present increased levels of DNase I as compared to the control strain MRL-+/+. This DNase I, which most probably originates from the accumulated CD4-CD8-T-cells, cleaves nuclear DNA with a strong preference for internucleosomal sites yielding, in the presence of both Ca2+ and Mg2+, a pattern of fragments typical for apoptosis. Furthermore, we show that this "apoptosis-ladder" can be obtained with purified DNase I in presence of normal serum.