P-glycoprotein consists of two homologous halves, each composed of six potential transmembrane sequences and an ATP-binding domain. The cDNA coding for human P-glycoprotein was divided in half and subcloned into separate plasmids in order to express each half as a separate polypeptide and to characterize its contribution to function. Expression of cDNAs coding for either the NH2- or COOH-terminal half-molecules in HEK 293 cells yielded products of 88 and 64 kDa, respectively. The NH2-terminal half-molecule was glycosylated, since its size decreased from 88 to 79 kDa when expressed in the presence of tunicamycin. No change was observed in the size of the COOH-terminal half-molecule when it was expressed in the presence of tunicamycin, indicating that it was not glycosylated. The cDNAs coding for each half of P-glycoprotein were transfected into NIH-3T3 cells to test for biological activity. No drug-resistant colonies were obtained when cells were transfected with cDNA coding for each half-molecule or when cells were co-transfected with both cDNAs, although stable expression of each half-molecule was detected. The inability to confer drug resistance was likely due to a defect in targeting of the half-molecules to the cell surface. Each half-molecule was then expressed in Sf9 insect cells using a baculovirus vector to allow measurement of partial function. The half-molecules exhibited ATPase activity, but their activities were not stimulated by drug substrates. Drug-stimulatable ATPase activity was present, however, when the half-molecules were expressed together. These results suggest that coupling of ATPase activity to drug binding requires interaction between both halves of P-glycoprotein.