Large tonsillar lymphocytes labeled with [3H]thymidine reverted to small lymphocytes with concomitant loss of [3H]DNA upon culturing. The decrease of labeled DNA content and size of large lymphocytes was demonstrated by flow cytometry and cell sorting. These observations suggest that stimulated lymphocytes may revert from their proliferative phase to resting phase by shedding 'extra'DNA under cell culture conditions. This released DNA is not due to cell damage and can be hybridized to chromosomal DNA.