The synthesis of DNA, as measured by the incorporation of [3H]-thymidine into the nuclei of cells of cultured explants of rabbit spleen, has been studied and correlated with estimation of DNA content of these nuclei using Feulgen densitometry, with and without colcemid arrest. Observations of the proportion of nuclei labelled with [3H]thymidine and their DNA content indicated that many cells were incorporating this precursor for DNA synthesis without the concomitant increases in DNA expected of cells in division cycle. Colcemid arrest failed to produce the changes in DNA content expected of a cell population as rapidly dividing as the labelling kinetics suggested. The results indicated that [3H]thymidine-labelled material, possibly DNA, was being transferred between cells and this accounted for the increased labelling index seen after pulsing with [3H]thymidine. The suppression of [3H]thymidine uptake by hydroxyurea indicated that this was not repair synthesis of damaged DNA.