An essential step in the replication cycle of retroviruses is the dimerization of two copies of the genomic RNA. In vitro and in vivo studies have demonstrated that dimerization is mediated at least partially by RNA-RNA interactions. In HIV-1, the cis-element most important for dimerization is the dimerization initiation site (DIS), a stem-loop structure with an autocomplementary loop located between the primer binding site and the splice donor site in the 5' leader region of genomic RNA. We have studied the inhibition of dimerization of RNA corresponding to the first 615 nt of HIV-1 strains MAL and LAI in vitro using RNA and DNA oligonucleotides. The oligonucleotides were identical to or complementary to the DIS of the MAL and LAI strains, which are representative of the two most common DIS motifs found in natural isolates. The loop sequence of the DIS of the MAL isolate is AGGUGCACA, and that of the LAI sequence is AAGCGCGCA (the autocomplementary sequences are GUGCAC and GCGCGC, respectively). Several of the oligonucleotides were very efficient inhibitors of dimerization. However, homologous oligonucleotides displayed vastly different inhibition efficiencies between the two strains despite relatively modest sequence differences. Some of the oligonucleotides bound the viral RNA via a loop-loop interaction alone, whereas others recruited stem nucleotides to form an extended duplex even in the absence of loop complementarity. Furthermore, oligonucleotide inhibition was ineffective at low temperature, suggesting that a conformational change in the DIS is necessary for disruption of the dimeric structure of the DIS or binding of oligonucleotide or both.