Abstract
In phytohaemagglutinin-stimulated lymphocytes, pulse labelled with tritiated thymidine for one minute, the acid-precipitable radioactivity was in the form of fragments that banded at the top of an alkaline sucrose gradient. When the radioactivity was chased with unlabelled thymidine for 2 hrs, most of the acid-precipitable radioactivity banded with the bulk of the DNA in the lower half of the gradient. On further chasing from 5 to 24 hrs, the radioactive DNA was fragmented and could be located on the lighter side of the gradients. This later fragmentation of radioactive DNA was associated with a loss of acid-precipi table radioactivity from the cells and appearance of DNA in the medium. After 24 hrs, approximately 67% of the acid-precipitable radioactivity was present in the medium. 60 to 70% of the radioactivity put out into the medium was acid precipitable, alkali resistant and DNase sensitive.