Type 1 diabetes (T1D) is an autoimmune disorder with a strong inflammatory component. Autoreactive cells specifically target insulin-producing ?-cells, which leads to loss of glucose homeostasis. T1D remains incurable and versatile; potentially beneficial therapeutics are being tested worldwide. Possible candidates for the treatment of autoimmune diabetes are plants and their extracts since they are rich in biophenols, substances that act as secondary metabolites, and have verified antioxidant and antiinflammatory effects. Salvianolic acid B (SalB) is a biophenol and one of the major constituents of Origanum vulgare ssp. hirtum (Greek oregano) extracts which in our previous studies was shown to exhibit an antidiabetic effect in mice. The aim of the present study was to determine whether SalB is responsible for the observed effects of Greek oregano extracts. SalB was applied in vitro to macrophages and lymphocytes isolated from C57BL/6 mice, as well as in vivo in the model of T1D induced by multiple low doses (MLD) of streptozotocin (STZ). SalB did not affect the viability of cells, but it significantly decreased secretion of nitric oxide (NO) and TNF in lipopolysaccharide (LPS)-stimulated macrophages, as well as the secretion of IFN-? in concanavalin A (ConA)-stimulated lymphocytes. However, when applied in vivo, SalB at a dose of 2.5 mg/kg b.w., applied for 10 consecutive days, failed to protect mice from diabetes development. In conclusion, SalB exerts immunomodulatory effects in vitro, but is not effective in prevention of T1D in vivo. It probably requires cooperation with some other substances for the maximum efficacy exhibited by oregano extracts.