Sydowinin B is a xanthone mycotoxin isolated from Aspergillus sydowii with immunosuppressive and weak cytotoxic activity. Interestingly, sydowinin B was isolated from a vast algal bloom which formed off the eastern coast of Australia following dust storms in 2009 with the potential to adversely affect photophysiological performance of the coral reef dinoflagellate endosymbiont Symbiodinium. Sydowinin B inhibits concanavalin A-induced (T-cell) and lipopolysaccharide-induced (B-cell) proliferation of mouse splenic lymphocytes with IC50 OF 19 and 21 µg/mL, respectively.