Effect of Tamoxifen on Antithrombin II activation in breast cancer patients. We investigated the effect of tamoxifen on Antithrombin II (AT III) activation in metastatic and early stage breast cancer patients. AT III activation was low in 33.3 percent of untreated metastatic breast cancer patients. The levels were low in very advanced cases especially in those having liver metastasis. There was no significant statistically difference in AT III activities between untreated and those under chemohormonal therapy with metastatic disease. In the latter group, AT III activities were also low in those having very extensive and progressive disease. There was no significant difference in AT III activities between those having early stage disease and these under adjuvant chemmohormonal therapy. This study shows that ATIII activation could be decreased in very advanced stage metastatic patients in the natural course of the diesase and neither tamoxifen nor cytotoxic therapy have no effect to decrease AT III activation.