Antisense oligonucleotides are potentially a powerful tool for the therapeutic manipulation of genes associated with cancer. However, pharmacological applications of oligonucleotides have been hindered by the inability to effectively deliver these compounds to their sites of action within cells. In this study, we have prepared peptide-oligonucleotide conjugates with the intent of improving intracellular delivery. The phosphorothioate oligonucleotide component of the conjugates was complementary to a site flanking the AUG of the message for P-glycoprotein, a membrane ATPase associated with multidrug resistance in tumor cells. Two types of peptide-antisense oligonucleotide conjugates, but not mismatched control conjugates, provided substantial inhibition of cell surface expression of P-glycoprotein. Surprisingly, the peptide-oligonucleotide conjugates were more potent in the presence of serum than when used under serum-free conditions; this is in striking contrast to most other approaches for intracellular delivery of nucleic acids. Effective inhibition of P-glycoprotein expression was attained with submicromolar concentrations of antisense conjugates under serum-replete conditions. The combination of relatively modest molecular size and good efficacy in the presence of serum proteins suggests that peptide-antisense oligonucleotide conjugates may have significant promise for in vivo therapeutic applications.