The mold Geotrichum lano-brunneum produces a complex of closely related compounds with broad-spectrum antifungal activity. The principal active component, A 25822B (molecular formula C28H45NO), was characterized by spectroscopic (UV, IR, NMR) data and chemical reactions (e.g., reduction with KBH4 to form an amorphous dihydro derivative). Its structure (la) was conclusively determined by X-ray crystallographic analysis of its methiodide derivative. A related minor factor, A 25822A, was shown to be the 4,4-dimethyl analog (lb) of A 25822B with the molecular formula C30H49NO. Compounds la and lb belong to a new family of mold metabolites and represent the first reported naturally-occurring homo-aza sterols. Details of their isolation, properties, and other factors of the complex will be published in subsequent papers.