Two tumor lines exhibiting different metastasizing capacity were studied with respect to the lymphocyte-induced angiogenesis (LIA). The MM3 line produce lung metastases with a high incidence. Conversely, the metastatic incidence of M3 line is much lower. Tumor cell suspensions were inoculated in BALB/c mice, 24 h later spleens were removed from the animals. Lymphocyte suspensions were made and then injected intradermally in syngeneic recipient mice. Five days after the injection of lymphocytes the vascular reaction was evaluated in the skin of the recipients by measuring the vessel density. The vascular response induced by lymphocytes from MM3-tumor bearing mice was increased when compared to that of M3-tumor-bearing mice. There appeared to be a positive correlation between metastasizing capacity and the lymphocyte-induced vascular response.