NM23 is a protein associated with tumor progression, expressed in all tissues and in human tumors. Reduced expression of NM23.H1 is related to high incidence of lymph node and distant metastasis or to poor prognosis of the patient in several human malignant tumors. In this study we analyze NM23 expression in non-neoplastic mammary tissues surrounding the tumoral lesions, in human mammary carcinomas and in lymph node metastasis. Our analysis shows that NM23.H1 expression is lower in the mammary cells surrounding the tumor than in the tumor itself. In the primary tumors we observed a negative trend between degree of local invasion and level of NM23.H1 expression. A further decrease of NM23.H1 was detected in the invasive tumors that metastasized to axillary lymph nodes and in the metastasis. NM23.H2 was always more highly expressed than NM23.H1, and reduced expression of NM23.H1 but not NM23.H2 was concordant with the presence of lymph node metastasis or local invasiveness of the primary tumor. A positive correlation between NM23.H1 mRNA content and cell growth rate of breast tumor cells has been confirmed. However, this trend was not maintained in cancer cells from tumors that metastasized to axillary lymph nodes and in metastatic cells; in these 2 situations the NM23.H1 mRNA content varied without any relationship to the proliferative rate of the cells. In addition, in comparison with the initial tumor, the metastatic cell population showed a strong decrease of NM23.H1 expression and increased proliferative activity.