The fungusCyathus helenaeBrodie (Gasteromycetes) produces a complex of antibiotic substances collectively called cyathin. Highest yields were obtained when the fungus was grown at room temperature for 25 days on a chemically defined liquid medium in static culture. The action spectrum of cyathin (and its components) was determined by placing cyathin-bearing paper discs on agar plates seeded with the microorganisms being tested. The cyathin complex is active against actinomycetes, bacteria (gram-positive as well as gram-negative), and some fungi, including dermatophytes. Weaker antimicrobial activity is also shown byC.striatus(Huds.) Willd. ex Pers.,C.poeppigiiL., andC.limbatusL. Seven different compounds have been isolated from the cyathin complex: 2,4,5-trihydroxybenzaldehyde (C7H6O4),cyathin A3(C20H30O3), cyathin A4(C20H30O4),allocyathinA4(C20H30O4),cyathin B3(C20H28O3), cyathin B4(C20H28O4), and cyathin C5(C20H26O5). Aside from 2,4,5-trihydroxybenzaldehyde, these compounds appear not to have been previously described. A discussion of some of the structural features of cyathin A4and cyathin A3is presented