Liver slices incubated in Ringer-Krebs-bicarbonate solution and repeatedly washed in the same fluid released about 60% of their content of CoA. CoA free liver slices were unable to take up acetate 1-C¹⁴ into fatty acids at the same rate as normal slices did. When these slices were incubated in presence of acetyl-1-C¹⁴ CoA, C¹⁴ labelled fatty acids were again synthetized. The uptake of radioactivity from the incubation fluid run parallel to the disappearing of CoA from the same fluid. Thus CoA seems to permeate in vitro cell membrane of liver slices although this condition is obviously strictly dependent on the particular pretreatment of the slices.