Massive chromatin fragmentation (around 50 kb, to several hundred kb) is observed in nuclear lysates of human peripheral blood lymphocytes (PBLs) upon their treatment with nuclease-free protein-denaturants. There is a consistent variation in the fragment size distributions that parallels the proliferative activity of the cells. Predominantly approximately 50 kb fragmentation is exhibited in samples from cells immunsuppressed via CD4 crosslinking, as opposed to the heterogeneous, higher molecular weight DNA of anti-TcR/CD3-, phytohemagglutininor concanavalin A-stimulated cells. Tritiated thymidine incorporated into DNA in the latter cultures can be detected in the approximately 50 kb band. Direct lysis of agarose-embedded, live cells in alkali+detergent also yields fragmented DNA, with a single-strand size of >/=50 kb. These data suggest that (i) the cells yielding fragmented DNA were alive at the time of DNA extraction, (ii) either regularly arranged, preformed nicks or hypersensitive sites may be present at every roughly 50-100 kb in the chromatin of PBLs, (iii) these sites or the fragmentation mechanism acting upon them, appear to be regulated in concert with the transit of cells between the resting and proliferative compartments.