The possibility ofusing oligodeoxynucleotides complementary to viral RNA or proviral DNA to inhibit the replication of human T-cell lymphotropic virus type III (HTLV-Ill) [the etiological agent of acquired imnunodeficiency syndrome (AIDS)] in cultured human cells was addressed by studying the association of 32P-labeled oligodeoxynucleotides with mammalian cellular components. The results indicated that exogenous oligodeoxynudeotides at 20 pM became associated with the membrane/cytosol fractions of the cell in amounts approximating 1.5 pM. Oligodeoynudeotides complementary to a region close to the tRNAL9 primer binding site on HTLV-M RNA and others complementary to HTLV-I mRNA donor or acceptor splice sites inhibited viral replication (assayed as reverse transcriptase) and gene expression (assayed as virus-encoded proteins p15 and p24) by as much as 95%. Use of control (random) oligodeoxynudeotides suggests that the antiviral effects were specific. Although these esults pertain to HTLV-I-infected cells in tissue culture, rather than to AIDS patients, they nevertheless point to a therapeutic potential of the complementary oligodeoxynudeotide ("hybridization competition" or "hybridon") approach in the treatment of patients with AIDS and AIDS-related complex.