By use of different activators and inhibitors, TOCP(tri-o-cresyl phosphate), PCMB (parachloromercury benzoate), NiCl2, Pb(NO3)2, HgCl2, Hg(NO3)2, eserine and sodium taurocholate, it is shown that the esterase in the cyst cells and in group I cells of the guinea pig thyroid probably are A-esterase isoenzymes. The activity in the majority of cyst cells is considerably stronger than in the other thyroid epithelial cells, and it is resistant to Hg inactivation. Neither esterase type nor intensity of the reaction product is altered in group I-II or cyst cells during vitamin A-deficiency. When a normal diet is given, the esterase in all thyroid epithelial cells is very sensitive to sodium taurocholate, while in cyst cells it is rather resistant to this inhibitor at vitamin-A-deficiency.