Western immunoblots of whole cell lysate and crude membrane extract of an in vitro selected sodium m-arsenite-resistant L. donovani strain revealed a 230-kDa protein identified by an anti-P-glycoprotein (Pgp) antibody. Immunofluorescence microscopy, using the same antibody, detected putative Pgp on resistant parasites. Overexpression of the putative Pgp was downregulated by verapamil. These results provided, possibly, the first evidence that (i) overexpression of Pgp-like protein occurs in arsenite-resistant Leishmania that are cross-resistant to structurally and functionally unrelated drugs and (ii) verapamil regulates drug sensitivity possibly by down-regulating Pgp expression in the arsenite-resistant Leishmania.