In vivo selection and characterization of a murine mammary tumor subline with high potential for spontaneous lymph node metastasis. A transplantable mammary adenocarcinoma, grown in Balb/c mice, with a marked enhancement in its draining lymph node metastatic ability (MM3LN), was obtained through an in vivo procedure from a variant tumor moderately metastatic to lymph nodes (MM3). Both MM3 and MM3LN presented a similar latency and tumor growth rate and reached the same tumor mean diameter at death. MM3LN tumor-bearing mice exhibited a larger mean survival time. The new variant showed a 2.5-fold higher incidence of tumor-draining lymph node metastases than MM3 line, with no differences in the incidence of lung metastases. Morphology as well as cytogenetic and in vitro adhesion properties were studied in order to characterize the new subline. This murine tumor model has potential application in the study of the metastatic process in lymphoid tissue. 