Acetate is the preferred substrate for long-chain fatty acid synthesis in isolated spinach chloroplasts.

1. Commercially available 2-14C]pyruvate and 2-14C]malonate were found to contain 3-6% (w/w) of 14C]acetate. 2. The contaminating 14C]acetate was efficiently utilized for fatty acid synthesis by isolated chloroplasts, whereas the parent materials were poorer substrates. 3. Maximum incorporation rates of the different substrates examined were (ng-atoms of C/h per mg of chlorophyll): 1-14C]acetate, 2676; 2-14C]pyruvate, 810; H14CO3-, 355; 2-14C]malonate, 19. 4. Products of CO2 fixation were probably not a significant carbon source for fatty acid synthesis in the presence of exogenous acetate.