We previously demonstrated inmast cell lines RBL2H3 and FMA3 that tryptophan hydroxylase (TPH) undergoes very fast turnover driven by 26S-proteasomes [Kojima, M., Oguro, K., Sawabe, K., Iida, Y., Ikeda, R., Yamashita, A., Nakanishi, N. & Hasegawa, H. (2000) (Tokyo) 2000,127, 121–127]. In the present study, we have examined an involvement of TPH phosphorylation in the rapid turn-over, using non-neural TPH.