Proliferative activity was estimated in 10 benign lesions and 47 cancers of mammary gland (mainly, invasive ductal carcinomas), and 14 metastatic lymph nodes by means of immunohistochemical determination of PCNA. Total PCNA expressing nuclei (PCNAtot) and nuclei strongly stained (PCNAstr) were counted. Proliferation index (PI) measured by counting PCNAstr nuclei has a higher correlation (r = 0.94, p < 0.000000) with PI estimated by means of 5-bromo-2-desoxyuridine in vivo than that obtained by counting PCNAtot nuclei (r = 0.77, p < 0.000000). Total and strong PCNA expression increased significantly when graded according to H.J.G. Bloom & W.W. Richardson. There is a clear correlation between PI of primary breast cancer and its metastases in lymph nodes (r = 0.89, PCNAtot and r = 0.90, PCNAstr), but proliferative activity in metastases was significantly lower than that of in primary tumors (p = 0.02 and p = 0.003). PCNA expressing tumor cells is a method of evaluation of proliferative activity in tissues which permits using archival paraffin samples.