The rate of sister chromatid exchanges (SCEs) and the profiles of proliferation (= number of first, second, and third cycle metaphases distinguishable by their staining patterns after BrDU substitution for one, two, or three rounds of DNA replication) have been estimated in three (resp. five) metastatic human breast tumor cell lines. Line MDA MB 231 exhibited SCE rates equalling the level of normal cells, while line MDA MB 435 exhibited a slight increase, and line MDA MB 468 a more distinct increase of the SCE rate over the baseline frequency of normal cells. By the BRDU labelling technique the lines 231 and 435 were shown to be rather fast proliferating in culture, while the line 468, and even more the lines 330 and 361 showed a much slower proliferation pattern. From these results it was concluded that increased SCE rates are rather a facultative than an obligatory feature of malignant cells, and that the Br-DU-labelling technique might be suitable for gaining additional information on proliferation characteristics of tumor cells lines.