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Yeast fatty acid synthetase catalyzes the synthesis of palmityl-CoA and stearyl-CoA from acetyl-CoA and malonyl-CoA. Acetyl-CoA serves as "primer" of the reaction cycle. The specificity of the fatty acid synthetase with respect to the "priming" substrate was studied on the purified enzyme.1. I n its function as primer acetyl-CoA was replaced by homologous saturated acyl-CoA derivatives (butyryl-CoA, capronyl-CoA, etc.). I n these experiments nonanoyl-CoA and d ecanoylCoA were good primers in low concentration range. This kind of fatty acid synthesis leads to normal end products.2. At higher concentrations of priming substrate an inhibitory effect became apparent with all long-chain acyl-CoA derivatives. With decanoyl-CoA the degree of inhibition was dependent on the concentration of malonyl-CoA. This was interpreted as competition between decaiioyl-CoA and malonyl-CoA.3. The rate of synthesis with ~~-3-hydroxydecanoyl-CoA and A2-decenoyl-CoA as prtmer was 25-50 times slower than with decanoyl-CoA. The rate-limiting step was the transfer of the acyl residue from CoA to the enzyme. The saturated acids C,,-C,s were isolated as end prcsducts of this synthesis.4. Derivatives of deoanoyl-CoA with ciubstitutions a t positions more distant from the Icarboxyl group were not so strongly depressed i n their priming activity, e.g. d3-decenoyl-CoA or 4-0x0-decanoyl-CoA. I n these cases the functional groups were retained during chain elongation. As reaction products unsaturated acids or 0x0 acids respectively were isolated.5. The enzymatic reduction of the double bond (A3-decenoyl-CoA) or the 0x0 grou11 (4-0x0-decanoyl-CoA) could be demonstrated only as a slow side reaction.