We have previously reported the identification and characterization of a prolactin-inducible protein (PIP) as well as the cloning of the gene encoding PIP in cultured human breast cancer cells. We now present three lines of evidence that the gene encoding PIP is also expressed by some human breast cancers in vivo: detection of PIP immunoreactivity in the serum of some breast cancer patients; immunohistochemical detection of PIP in breast cancer sections; and the presence of PIP mRNA, detected by complementary DNA hybridization in human breast biopsy samples. In a preliminary study using Western blot analysis authentic PIP was detected in the serum of some patients with breast cancer. Subsequently the sera of 234 unselected patients with breast cancer were assayed for the presence of PIP using a specific radioimmunoassay. Thirty-five % of these sera contained detectable PIP (i.e., greater than 3 ng/ml). As well as were able to show by immunohistochemical techniques that PIP immunoreactivity was present in some human breast biopsy specimens. Levels of estrogen receptor, progesterone receptor, and PIP mRNA were determined in an unselected population of 51 human breast tumor biopsies. Sixty-one % of these tumors had detectable PIP mRNA; a positive correlation (r = 0.52; P less than 0.01) was found between PIP mRNA levels in breast biopsy samples and estrogen receptor content, a known prognostic indicator in human breast cancer.