Rat basophilic leukemia (RBL) cells were grown as tumors. Membrane vesicles were isolated from them and serotonin transport was measured. Two types of transport were identified. One was sensitive to imipramine and sodium-dependent, while the other was sensitive to reserpine and ATP-dependent. The transport systems exhibit different affinities for serotonin (sodium-dependent, Km 0.22 microM; ATP-dependent, Km 2.6 microM) and are different in their substrate specificity, the former being much more specific. The 5-hydroxytryptamine transport by the reserpine-sensitive system was strongly inhibited by other biogenic amines, like norepinephrine, epinephrine and dopamine, whereas that by the imipramine-sensitive system was not. Upon Ficoll gradient centrifugation the two transport systems were separated. The reserpine-sensitive activity was found much further into the gradient than the imipramine-sensitive one. The latter co-migrated with the receptor of IgE, which is located in the plasma membrane. Characterization of latter showed that in addition to the dependence of 5-hydroxytryptamine influx on external sodium it was also absolutely dependent on external chloride and was strongly stimulated by internal potassium. On the other hand, efflux required external potassium. An alternative potassium independent way of loss of labelled 5-hydroxytryptamine was by exchange. A small but consistent stimulation of influx was observed in the presence of valinomycin, indicating that the process is electrogenic. The reserpine-sensitive system could also be driven in the absence of ATP. This required the imposition of pH gradient (acid inside) and was stimulated by an artificially imposed membrane potential (positive inside).