After treatment of cells with 5‐bromodeoxyuridine (BUdR), the percentage of completely BUdR‐labelled interphase nuclei is greater the longer the BUdR treatment. The labelling effect is visible after staining with the fluorochrome 33258 Hoechst and with Giemsa. Various formulae and a nomogram are presented by means of which the percentage of cells in S period, duration of the S period and the whole cell cycle can be determined by examination of a single preparation or by comparison of several preparations. The methods are tested using cell cultures of Microtus agrestis and compared with autoradiographic methods after labelling with 3H‐thymidine.

A method of labelling DNA in vivo with 5-bromodeoxyuridine (BrdU) is described. After 6 h permanent subcutaneous infusion of BrdU in rodents (adult Microtus agrestis, pregnant NMRI-mice), cell nuclei which have undergone DNA synthesis during the BrdU treatment can be differentiated from the nuclei of other cycle stages by means of their altered staining behaviour after Giemsa. 24 h after the BrdU treatment, mitoses from both bone marrow of the adult animals and tissues from the fetuses showed a differential sister chromatid staining. In male M. agrestis, sister chromatid exchanges were most frequently found in the euchromatic part of the X and in the constitutive heterochromatin of both sex chromosomes.