We studied with computerized image analysis 236 breast cancer samplings after in vitro bromodeoxyuridine incorporation and immunohistochemical revelation. Labeling index values were compared with the usual prognostic factors and with the other studies in the literature. We established a positive correlation between labeling index and tumor size, histoprognostic grading, phase S and DNA index. A high labeling index was correlated with the absence of hormonal receptors but not correlated with the other prognostic factors. These results on tumor kinetics are similar to those obtained by flow cytometry and from other studies in the literature. However, this technic using optical microscopy allows for reliable selection of tumoral cells. Furthermore, the semi-automated image analysis provides an objective and reproducible evaluation of the labeling index.