Thirty-two metastatic breast cancer patients entered a phase II study in order to evaluate the toxicity and the clinical activity of lymphoblastoid interferon (IFN). The treatment was divided into two phases, induction and maintenance. During the first, patients were submitted to lymphoblastoid IFN at the dose of 3 MU die intramuscularly for 4-8 weeks; during maintenance treatment IFN was given at the same dosage, intramuscularly, every other day until progression. Five patients with inadequate follow-up were excluded from disease evaluation, thus 27 patients were evaluable for response. No complete response was observed; one patient achieved a partial response and 15 patients disease stabilization. Median time to progression in patients with partial response or stable disease was 11 weeks (range 2-32). Five patients with soft tissue metastases were biopsied before and after 1-2 months of treatment in order to perform Labelling Index and hormonal receptor status determinations. No significant modification was observed.