Cultured breast cancer cells express on their surface glycoproteins which are recognized by the peanut lectin (PNA). The proliferation of these cells (ZR-75.1 and 734-B) was inhibited by PNA. A mammary carcinoma cell line (BT-20) which does not react with PNA was not affected by this lectin. The combination of PNA with either retinoic acid or 4-hydroxy-tamoxifen led to an additive amplification of the antiproliferative activity. Also interferon-γ showed in combination with PNA an improved growth-inhibitory action.