We have determined serum copper levels in BALB/c female mice subcutaneously inoculated with two related mammary adenocarcinoma tumors. Serum copper content increased progressively with tumor size in animals bearing either variant, and reached levels up to four times those of control mice. In contrast, copper levels detected by cytochemistry in tumor cells are higher in the slow growing tumor variant. It is suggested that the stronger angiogenic effect previously described for this variant could be related to its higher cellular copper content.