Results suggest that cytotoxicity by cells from nodes regional to a primary tumor is unique. While a primary tumor was present, A) cytotoxicity was displayed by cells from lymph nodes (RLNs) to that tumor, B) cells from nonregional lymph nodes regional (NRLNs) possessed lesser cytotoxicity which failed to increase in response to a second tumor focus in an area drained by those nodes, and C) the second focus attenuated cytotoxicity of cells from LNs regional to the primary tumor. Following removal of the primary tumor, cells from RLNs rapidly lost cytotoxicity and with passage of time were unable to regain that function in response to a second tumor focus. In contrast, cells from NRLNs demonstrated increased cytotoxicity at any time following removal of the primary tumor when exposed to a second focus. These observations suggest that nodes regional to a distant metastatic focus may be unable to react to it and thus contribute little to the host response generated by the primary tumor. In addition, since nodes regional to a primary tumor manifest diminished cytotoxicity in the presence of a distant tumor focus, tumor cells gaining access and lodging in those nodes subsequent to the development of other metastatic foci are likely to proliferate, resulting in the "positive" lymph node. The findings are in keeping with our contention that host factors responsible for metastases, and perhaps metastases themselves, are at least in part responsible for growth of tumor in RLNs. They also have relevance to the site of administration of specific immunotherapeutic agents and to the significance of the removal of RLNs with a primary tumor.