Interleukin-12 (IL-12) is known to be a key cytokine for regulating immune response, but it is also known to provide some other biological function including inhibition of angiogenesis. We have determined using an enzymatic immunoassay the endogenous levels of IL-12 in 390 cytosols of primary breast cancers previously tested also for the angiogenic peptides, vascular endothelial growth factor (VEGF) and thymidine phosphorylase (TP). The concentration of IL-12 ranged from 0 to 7.6 ng/mg protein, and 124 (31.8%) out of 390 cancers showed a detectable dose (>0.1 ng/ml). There was no statistical association of IL-12 levels with tumor size and menopausal status. IL-12 levels tended to be higher in the tumors of node-positive patients as compared to those of node-negative ones (t-test, p=0.082). In addition, IL-12 levels were inversely associated with hormone receptor status, particularly progesterone receptor expression (p=0.0013). There was a significant inverse association between IL-12 and TP concentration (p=0.0007). The proportion of tumors with detectable levels of IL-12 and low levels of either VEGF or TP was higher among the patients with node-negative as compared to those with node-positive disease. On the contrary, the proportion of tumors with no detectable IL-12 and high levels of either VEGF or TP was higher in node-positive versus node-negative cancers. In conclusion, our study evaluated the balance between pro-angiogenic factors (TP and VEGF) and IL-12, as a detectable naturally occurring inhibitor of angiogenesis, in the same series of node-negative and node-positive breast cancers. Further studies are warranted to investigate the biological and clinical significance of the co-determination of pro and contra angiogenic factors in human breast carcinoma.