Xanthoquinodin B3, a new component of anticoccidial xanthoquinodins, which was detected in the culture broth of Humicola sp. FO-888, was isolated by heat treatment of the xanthoquinodins complex. Structural elucidation indicated that xanthoquinodin B3 has the same heterodimer of xanthone- and anthraquinone-derived monomers as other xanthoquinodins. Schizont formation of monensin-resistant Eirner ia tenella in BHK-21 cells was inhibited by xanthoquinodin B3 at concentrations greater than 0.035 mu M.