In order to further characterize the different repairing behavior of thymus and spleen cells of rats in vitro under the influence of X-rays, UV radiation and methylmethanesulfonate (MMS), the effect of bleomycin (BM), L-cysteine (CY-E), N-ethylmaleimide (NEM), 1-beta-D-arabinofuranosylcytosine (araC), dideoxythymidine (ddT), and novobiocin (NB) on the semiconservative and restorative DNA synthesis as well as on the behavior of DNA under the alkaline elution was studied. The semiconservative DNA synthesis was inhibited by all examined agents except ddT, the restorative DNA synthesis only by NEM, araC, and NB. The stimulation of the restorative DNA synthesis was increased by UV radiation and MMS in spleen cells and by X-rays, BM and CY-E in thymus cells. Under the conditions of alkaline elution, there was a more sensitive reaction of spleen cells than of thymus cells to X-rays, BM and CY-E. The results show that thymus cells are especially qualified for the repair of short chains and spleen cells for the repair of long chains.