Abstract 1. Sedimentation in gradients of alkaline sucrose has been used to observe the production and rejoining of X-ray-produced strand breaks in the DNA of cells from rats and chickens. The DNA was estimated by a fluorimetric method which permitted the study of non-dividing cells whose DNA could not be labelled with radioactive precursors. 2. We have found that cells from the thymus or spleen of rats contain DNA of high molecular weight (> 10 9) and can rejoin single-strand breaks introduced into DNA by X-rays. This suggests that cells which are not synthesizing DNA possess at least one class of repair enzymes. 3. Cells prepared from muscle of 1-day-old rats contain high molecular weight DNA and can rejoin single-strand breaks. Muscle cells prepared from older animals (1 week or older) contain DNA of lower molecular weight and cannot rejoin strand breaks. This could be the result of the preparative procedure and not reflect the situation in vivo. 4. Newly formed chicken erythrocytes contain DNA of high molecular weight but cannot rejoin strand breaks. As the cells age in vivo, the molecular weight of the DNA decreases to about 2 · 10 8. This demonstrates that at least one type of postmitotic cell does not possess repair enzymes.