A polyoxomol ybdoeuropate PM-104 (NH₄)₁₂H₂[Eu₄(MoO₄)(H₂O)₁₆(Mo₇O₂₄)₄]·13H₂O was found to be a potent inhibitor of the growth of human immunodeficiency virus type 1 (HIV-1), a causative agent of acquired immunodeficiency syndrome (AIDS). On the basis of TI₅₀ [median cytotoxic concentration (CC₅₀)/median effective concentration (EC<₅0>)], the in vitro anti-HIV-1 activity of PM-104 is favorably comparable to that of a heteropolyoxotungstate PM-19 K₇[PTi₂W₁₀O₄₀]·6H₂O, which is one of the most potent HIV-1 inhibitors among the polyoxometalates so far tested. The heteropolyoxomolybdate with a potent anti-HIV-1 activity is introduced for the first time in this communication.