ACP was identified initially as a heat-stable protein required for fatty acid synthesis in extracts of Clostridium kluyveri and E.~ 0 li. l~ The significance of this protein was established by the early demonstration that the product of the condensation of acetyl CoA and malonyl CoA is acetoacetate bound as an acyl group to ACP. 4.'4 This corroborated earlier reports by F. Lynen15. 16 which demonstrated that acetoacetyl-protein is formed from acetyl CoA and malonyl CoA in a yeast fatty acid synthesizing system. Isolation and purification of acetoacetyl-ACP in the E. coli system permitted the demonstration that acetoacetyl-ACP is a thioester and that acetoacetyl-ACP is rapidly incorporated into long chain fatty acids by the fatty acid synthesizing system." I4 Thus the acyl carrier function of ACP was established. ACP was purified approximately 400-fold from extracts of E. coli. The homogeneity of this preparation was demonstrated by the following: a single protein peak with constant specific activity throughout the peak upon column chromatography on DEAE Sephadex (FIGURE l), a single protein band upon starch gel electrophoresis, and a single symmetrical peak upon analytical ultracentrifugation (FIGURE 2). The sedimentation constant of ACP, S20, u= 1.44, and the diffusion constant, D20, u,= 13.5 x cm2 sec;'were determined and the molecular weight calculated from these constants is 9,750. Minimum molecular weight determined from the amino acid analysis is 9.488. 6. R