The participation of polyketides in the biogenesis of natural products has long been bolstered by chemical analogies. Many isotopic tracer studies have validated the acetate-polymalonate route, via presumptive extended poly-8-carbonyl intermediates, to a variety of fungal metabolites. Though implicit as antibiotic precursors, the ephemeral polyketides have not been isolated, nor perhaps with the exception of acetoacetate, can oligoketides become incorporated intact into secondary metabolites. However, a prototypical oligoketide in its stable lactone form, methyltriacetic lactone (3,6-dimethyl-4-hydroxy-2-pyrone), has been obtained from the tropolone-producing mold P. stipitatum. A convenient synthesis of this metabolite, by methylation of triacetic lactone followed by partition chromatographic separation of the resultant positional isomers, has been devised. In an experiment with 14C-formate, it was shown that the hypothetical, enzyme-bound polyketide precursor to methyltriacetic lactone is probably involved in stipitatic acid formation, and that the origin of the "extra" methyl or methyl-derived carbons of both substances arises from the identical "C1" pool. Radioactive tracer experiments concerning the biogenesis of pulvilloric acid, a fairly unstable antibiotic substance produced by P. pulvillorum, showed that its exocyclic carboxyl is formed following initial methyl transfer, whereas the ring system of the molecule is essentially acetate-polymalonate derived. In order to test the hypothesis that methyl-branched C, polyketide precursors t o pulvilloric acid exist and may become integrated into the fatty acid multienzyme complex, presumptive fatty acid congeners to pulvilloric acid such as 4-methylmyristic, Cmethyllauric, or 2-methyllauric acids were sought. These substances were, however, absent from the mycelial fatty acid spectrum, as well as from the fatty acid moieties of a crystalline glyceride mixture obtained from the beer. Alternative approaches to the detection or isolation of polyketides are discussed.