1. Comparative kinetics and product analyses of chicken and rat liver and yeast fatty acid synthetase. 2. Vmax's for the three enzymes studied decrease with increasing primer chain-length, while Km's (except for yeast) increases. 3. Palmitate is the main product (approximately 90%) of the rat synthetase whereas palmitate (60%) and stearate (40%) are the products of chicken and yeast enzymes. Increasing the primary chain length does not alter palmitate synthesis by rat and chicken enzymes but increases stearate synthesis by the yeast synthetase. 4. The three synthetases could not synthesize fatty acids in the absence of primer acetyl-CoA suggesting that malonyl-CoA decarboxylase is not a component.