For many years, we have been studying, in psychiatric conditions, the influx of tyrosine (TYR) and tryptophan (TRP), the two amino acid precursors of monoamines, across the membrane of human blood cells. We have also attempted to characterize better the transport mechanisms. In a previous paper, we suggested a close relationship between glucose and the two neuter amino acid transports in vitro. The purpose of the present study is to test the effect of cytochalasin B, the specific and potent inhibitor of glucose transport. Our data show that at high concentrations, the cytochalasin B induces a reversible inhibition of about 70% or more on the temperature-dependent influx of the two amino acids, depending on the medium of incubation. The effect of cytochalasin B was about 200 times less for TYR and TRP transport than for glucose. The cytochalasin E, claimed to be a nonspecific inhibitor, decreased both these transports only when used at very high concentrations, as described for sugar influx in the same structure. In conclusion, we suggest that there is a relationship between the transport of glucose and nucleosides, both carried into the cells by the glycoprotein band 4.5, and the two amino acid precursors of monoamines.