Interleukin-35 (IL-35), is a recently identified cytokine that belongs to the IL-12 family, it is a potent antiinflammatory and immunosuppressive cytokine which was first recognized to be produced by regulatory T cells (Tregs) cells, and recently was found to be produced by regulatory B cells (Bregs). The study aimed at determining whether plasma levels of IL-35 are associated with clinical characteristics of breast cancer (BC) patients. The study included 40 patients with breast cancer (BC), and 10 matched controls. The IL-35 cytokine was measured in plasma using ELISA. Results showed that plasma IL-35levels were significantly higher in BC than healthy controls (P˂ 0.05), and were significantly associated with BC grade 2 and HER-2 over expression level “3+”, suggesting that plasma IL-35 levels may be associated with the development and progression of BC.