The growth and lipid metabolism of the mycelium of A. bisporus were investigated. A 30% stimulation of mycelial growth was observed upon the addition of small amounts of sodium acetate or linoleic acid to a complex growth medium. Lipid metabolism was studied with the addition of isotopically labeled sodium acetate or linoleic acid to the growth medium. Label was incorporated similarly into mycelial lipid from either tracer and in proportion to its mass. This suggests that linoleate is degraded to acetate units prior to use in lipid synthesis. The lipid of mycelium grown with sodium acetate or linoleic acid added to the medium contained 40% more linoleic acid than the lipid of nonsupplemented mycelium. Sodium acetate probably acts as a micronutrient stimulating an enzyme or enzymatic pathway leading to the synthesis of linoleate.