Human lactoferrin has been found to be decreased or absent in most breast cancer and leukemia cells. In order to examine the lactoferrin gene for both structural alterations and the degree of methylation, we isolated a 2117-kilobase complementary DNA from human breast tissue. This complementary DNA was used to probe DNA extracted from normal peripheral blood, leukemia cells from patients, leukemia cell lines, and breast cancer cell lines. Immunocytochemical staining of these cells confirmed the decreased production of lactoferrin in malignancy. MspI restriction enzyme fragment patterns demonstrated genetic polymorphism which occurred in DNA from both normal and malignant cells. Polymorphism was also noted with XbaI. In this case, there were two fragment patterns that were only found in DNA from malignant cells. The degree of DNA methylation was also evaluated. The methylation pattern of DNA extracted from malignant cells was highly variable and generally less methylated than DNA extracted from normal WBCs. It is possible that the decrease in lactoferrin associated with cancer is multifactorial and includes gene structural changes as well as altered regulation. Further study is needed to determine whether the changes found in this study are the result of the malignancy or contribute to its onset or maintenance.