OUTLINE: This is a randomized study. Patients are stratified according to disease free interval from primary diagnosis to first metastases (less than 3 years vs 3 years and longer), estrogen receptor status (positive vs negative), and disease status (complete response, partial response, detectable disease, or stable disease). Patients are randomized to one of two treatment arms. Arm I: Patients begin therapy no sooner than 3 weeks and no later than 6 weeks since last chemotherapy dose. Patients receive interleukin-12 subcutaneously twice a week. Treatment continues in the absence of disease progression or unacceptable toxicity. Patients are followed at least every 3 months for 1 year. If no progression after 1 year, may be followed as needed for new signs or symptoms and survival for 5 years. Arm II: Patients are observed for 6 months. If disease progresses during first 6 months, patients may receive interleukin-12 as in arm I. Patients without disease progression within first 6 months may also then receive interleukin-12 as in arm I. Treatment continues in the absence of disease progression or unacceptable toxicity. Patients are followed for toxicity only until interleukin-12 is discontinued.