We have previously described the potent and selective inhibition of several strains of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2) by JM2763, an n-propyl-linked dimer of the 1,4,8,11-tetraazamacrocyclic (cyclam) ring system. Upon further investigation, we have also found that incorporating an aromatic rather than aliphatic linker leads to analogs with higher antiviral potency. The prototype, JM3100 (19a, isolated as the octahydrochloride salt), which contains a p-phenylenebis(methylene) moiety linking the cyclam rings, inhibited the replication of HIV-1 (IIIB) and HIV-2 (ROD) at EC50's of 4.2 and 5.9 nM, respectively, while remaining nontoxic to MT-4 cells at concentrations exceeding 421 microM. In order to identify the structural features of bis-tetraazamacrocycles required for potent activity, we have prepared a novel series of phenylenebis(methylene)-linked analogs, in which the macrocyclic ring size was varied from 12 to 16 ring members. Depending upon the substitution of the phenylenebis(methylene) linker (para or meta), sub-micromolar anti-HIV activity was exhibited by analogs bearing macrocycles of 12-14 ring members but with varying cytotoxicity to MT-4 cells. Furthermore, while we found that identical macrocyclic rings are not required for activity, substituting an acyclic polyamine equivalent for one of the cyclam rings in 19a resulted in a substantial reduction in anti-HIV potency, clearly establishing the importance of the constrained macrocyclic structure. A short series of transition metal complexes of 19a were also prepared and evaluated. Complexes of low kinetic stability such as the bis-zinc complex retained activity comparable to that of the parent compound. Finally, the activity of bicyclam analogs appears to be insensitive to the electron-withdrawing or -donating properties of substituents introduced onto the linker, but sterically hindering groups such as phenyl markedly reduced activity. As a result, several analogs with anti-HIV potency comparable to that of 19a have been identified.