Comparative investigations on two morphologically diverse but genetically isologous mammary tumors of mice (DBA/212) revealed differences in the cell cycle ( Goldfeder , 1965). In the following paper, the cell kinetics of an anaplastic and a partly differentiated transplantable mammary carcinoma of mice have been studied. Material and Methods: The partly differentiated mammary carcinoma HB (Dr. C. W. Friis , Ry, Denmark) was examined in 55 female mice (C3H/Tif/Bom, 20–30 g), the anaplastic mammary carcinoma (Institute of Experimental Pathology, Farbenfabriken Bayer), derived from a spontaneous breast tumor of a C3H mouse, has been investigated in 55 female mice (NMRI, 20–30 g). For transplantation the subcutaneously growing tumors were removed in toto under sterile conditions, minced and stirred up in 5 ml isotonic saline. 0.5 ml of the tumor suspension was injected subcutaneously in the lateral flank. The autoradiographic experiments started on the 7th day after transplantation and randomization. Autoradiography: The cell cycle was determined by the method of percentage of labeled mitoses ( Quastler and Sherman , 1959). After a single intraperitoneal 3H-thymidine-injection (2 μ/g body weight, specific activity 2 m Ci/mole NEN Clemical GmbH), 1–4 animals were killed by cervical dislocation at 1 hour intervals (duration of the experiment up to 28 h). The tumors were fixed 24 h (neutral 6% formol with admixture of 0.1 mg/ml inactive thymidine) and embedded in paraffin. 5 μ tissue sections of the marginal area of the tumors were taken and prepared for autoradiographs (K2- and G5-emulsion, Ilford, London), exposed for 7 (G5-emulsion) or 21 days (K2-emulsion) and the developed autoradiographs stained with hematoxylin-eosin. Counts of 100–200 mitoses (late prophase to early telophase) were made for the estimation of percentage of labeled mitoses in each tumor. For determination of the approximate growth fraction by the continuous labeling method ( Mendelsohn , 1960 a and b, 1962) the animals received every 4 hours 3H-thymidine intraperitoneally (2 μ Ci/g body weight; Lobbecke and Schultze , 1966; Hempel , 1968; Blenkinsopp , 1969). Afterwards 2–3 animals were killed at 1 or 2 h intervals (experimental time up to 17 h). To provide a more precise count of the labeling index especially in the estimation of the growth fraction, besides tissue sections 4 air-dried smears of the marginal areas of each tumor were washed in ethanolglacial acetic acid (3 : 1) with an admixture of 1 mg/ml inactive thymidine, fixed and the autoradiographs prepared according to the method described above. Results and Discussion: The histological features of both breast carcinomas were as follows: the partly differentiated tumor (mammary carcinoma HB) transplanted on C3H mice showed an adenoid pattern with solid alveolar areas (Fig. 1a). Furthermore, squamous metaplasia with keratinization was present. The anaplastic carcinoma transplanted on NMRI mice exhibited a diffuse infiltrating growth with pronounced pleomorphism of cells and a tendency to necrosis (Fig. 2). Examining the autoradiographs it was striking that the anaplastic carcinoma was diffusely labeled especially in the marginal zone, whereas the partly differentiated carcinoma showed a more spotted labeling in areas looking like proliferating center (Fig. 1b). Mammary carcinoma HB: Fig. 3 a shows the percentage of labeled mitoses at various time intervals after a single injection of 3H-thymidine for the partly differentiated breast carcinoma. The first labeled mitoses were visible 1 h after injection (tG2 min). After 4 h approximately all mitoses were labeled. Taking the time interval between the midpoints on the ascending and the descending slopes of the first maximum the DNA synthesis phase (= tS) was 7.5 h. The minimal generation time was 13 h and the most frequent generation time (= tC) 15.5 h. Anaplastic mammary carcinoma: In fig. 3 b the percentage of labeled mitoses has been plotted against various time intervals after a single 3H-thymidine injection for the undifferentiated mammary carcinoma. The first labeled mitoses appeared 1 h after injection (= tG2 min). After about 6 h the maximum of the percentage of labeled mitoses was found. According to the 50% width of the first maximum in fig. 3 b, tS was 7.5 h; the minimal generation time corresponding to the labeled mitoses curve was 15 h; the most frequent generation time (= tC) was not exactly determinable, it should be about 16 h. Mammary carcinoma HB: Fig. 4 a depicts the percentage of labeled cells as a function of experimental time after repeated injections of 3H-thymidine (4 hour intervals). At the time (tC-tS) (= 8 h) about 55% of tumor cells were labeled, i. e., about 55% of tumor cells have a generation time of 15.5 h found by the percentage of labeled mitoses method. Therefore the approximate value of the growth fraction should be about 55%. Anaplastic mammary carcinoma: Fig. 4 b gives the percentage of labeled cells as a function of time after repeated injections of 3H-thymidine (4 hour intervals) for the undifferentiated carcinoma. At the time (tC-tS) (= 8.5 h) 50% of the tumor cells were labeled so that about half of the tumor cell population has a generation time of about 16 h determined by the percentage of labeled mitoses method. The approximate value for the growth fraction should be about 50%. In summary both transplantable carcinomas, showing a different microscopical picture, show no remarkable difference in the generation cycle and the growth fraction. These results are in agreement with preliminary in-vitro results for rapidly growing human cells which showed relatively small fluctuations in the cell cycle and DNA synthesis time. This might be important for the timing of cytostatic therapy (p. e., synchronization therapy).