Of the approximately 18,000 new cases of cancer in Venezuela each year, only half can be treated with surgery and radiation. The remainder must be treated systematically using chemotherapy or biological response modifiers. It has become evident that any drug resistant human tumors express the MDR1 gene, since MDR1 RNA levels are elevated in many cancers that do not respond to chemotherapy. Human mammary carcinomas have multiple oncogene alterations, the most frequently reported being overexpression of the oncogenes c-myc, int-2, neu and c-myb. Thirteen specimens of mammary cancer were obtained by biopsy of untreated patients in stage IIIB. All these patients received three cycles of FAC or CMF-L+GM-CSF after biopsy. In the slot blot analysis of RNA from invasive carcinomas, MDR1 and c-myc transcripts were detectable at a high level in 30% of tumors. Two patients with increased levels of MDR1 before chemotherapy did not respond to the treatment and distant metastasis and death occurred in these patients. Another patient, MDR1-negative before therapy, did not respond to CMF-1 + GM-CSF and showed high levels of MDR1 transcripts in a second biopsy which was obtained during surgery.