Mice were given 4 injections of [3H]thymidine at 2-hourly intervals and killed at times varying from 1 h to 12 days after the last injection. Autoradiographs of spleen, oesophagus and heart were prepared by the stripping-film technique and exposed for 21, 189 and 367 days.

Groups of lymphoid cells surrounding trabeculae were heavily labelled, the decline in grain count suggesting division of these cells within 24 h. The appearance of large numbers of labelled nucleated cells in venules suggests that cells leave the spleen through the lining of the venules. The cells in germinal centres were preponderantly weakly labelled and spread into the white pulp after 2 days. It is suggested that the weak labelling indicates turnover of DNA rather than pre-mitotic synthesis. Megacaryocytes were unlabelled 1 h after a single injection of [3H]thymidine, nearly all being labelled from 1 to 8 days after the last of 4 injections. The grain count declined steadily with increasing interval between injection and fixation.

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