The biological effects and cellular uptake of human c-myc antisense oligonucleotides and their liposome complexes were investigated in vitro using human promonocytic leukemia U937 cells. Antisense phosphorothioate oligonucleotides (S-Oligo) significantly inhibited the growth of U937 cells in a dose-dependent manner. However, no significant effect on cell proliferation was observed with unmodified phosphodiester (P-Oligo) and partially phosphorothioated (PS3-Oligo) oligonucleotides with an antisense sequence and S-Oligo with sense and G-quartet control sequences. In cellular uptake experiments, radiolabeled S-Oligo was taken up by U937 cells more than P-Oligo and PS3-Oligo. Similar results were obtained in mouse peritoneal macrophages used for comparison. Confocal microscopic studies demonstrated a significant distribution of FITC-labeled oligonucleotides on the cell surface and in the cytoplasm in a punctate pattern, but not in the nucleus. When complexed with cationic liposomes, cellular uptake of FITC-labeled P-Oligo or S-Oligo was significantly increased and the fluorescence was located mainly in the nucleus, indicating that the uptake and intracellular pharmacokinetics of both oligonucleotides can be modified by complexation. An inhibitory effect of the complexes was observed at a dose which is ineffective in the case of the oligonucleotides alone. However, this effect was also associated with cytotoxicity of the cationic liposomes, suggesting that optimization of this formulation will be necessary to achieve a more efficient delivery of the oligonucleotides to U937 cells.