The transport mechanism of PaA into human erythrocytes and lymphocytes from rabbit appendix was investigated. 1. The transport of pantothenic acid (PaA) into human erythrocytes was considered to be performed by simple diffusion mechanism. There was no necessity of energy or particular carrier protein for the transporting mechanism. 2. In the case of lymphocytes, the PaA transporting mechanism was seemed to be different from that of erythrocytes. When PaA concentration in the reaction medium was elevated, the incorporated amount of the vitamin into lymphocytes reached to some saturated level. This incorporation was inhibited by high concentration of dinitrophenol. However, the effect of addition of glucose was not so remarkable. Consequently, it is difficult to decide whether the transport reaction of PaA into lymphocytes is active or passive.