Previous work conducted in our laboratories established the notion that TPGS 1000 (d-alpha-tocopheryl polyethylene glycol 1000 succinate), a nonionic surfactant, modulates P-glycoprotein (P-gp) efflux transport via P-gp ATPase inhibition. The current in vitro research using Caco-2 cells was conducted to further explore the P-gp ATPase inhibition mechanism. Using a monoclonal CD243 P-gp antibody shift assay (UIC2), we probed P-gp conformational changes induced via TPGS 1000. In the presence of TPGS 1000, UIC2 binding was slightly decreased. TPGS 1000 does not appear to be a P-gp substrate, nor does it function as a competitive inhibitor in P-gp substrate efflux transport. The reduction in UIC2 binding with TPGS 1000 was markedly weaker than with orthovanadate, data ruling out trapping P-gp in a transition state by direct interaction with one or both of the P-gp ATP nucleotide binding domains. An intracellular ATP depletion mechanism could be ruled out in the UIC2 assay, and by monitoring intracellular ATP levels in the presence of TPGS 1000. Indicating slow distribution of TPGS 1000 into the membrane, and in agreement with an intramembranal or intracellular side of action, Caco-2 cell monolayer experiments preincubated with TPGS 1000 produce stronger substrate inhibitory activity than those conducted by direct substrate and surfactant coapplication.