The quantitative determination of CoA by sorbyl-CoA formation by Wakil et al. was investigated and applied to determination of CoA in animal tissues. Fatty acid activating enzyme (FAAE) which forms the sorbyl-CoA from sorbate, CoA and ATP was prepared mainly according to Mahler et al. The preparation method was a little modified and simplified by starting mitochondrial materials prepared from beef liver according to Hogeboom. The enzyme activities obtained were reproducible, but the range of optimal CoA determination was to the extent of 9.38 mμ moles. The extraction of CoA from animal tissues was as follows : homogenize at 0℃, boil after adjusting pH to 4.0 and add NaCl (2.5%) and centrifuge in the cold. This method was considered to be suitable for determination of CoA in animal tissues, although the preparation of active FAAE was somewhat difficult.