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The gene encoding an integrase of Mason–Pfizer monkey virus (M-PMV) is located at the 3′-end of the pol open reading frame. The M-PMV integrase has not been previously isolated and characterized. We have now cloned, expressed, isolated, and characterized M-PMV integrase and compared its activities and primary structure with those of HIV-1 and other retroviral integrases. M-PMV integrase prefers untranslated 3′-region-derived long-terminal repeat sequences in both the 3′-processing and the strand transfer activity assays. While the 3′-processing reaction catalyzed by M-PMV integrase was significantly increased in the presence of Mn2+ and Co2+ and was readily detectable in the presence of Mg2+ and Ni2+ cations, the strand transfer activity was strictly dependent only on Mn2+. M-PMV integrase displays more relaxed substrate specificity than HIV-1 integrase, catalyzing the cleavage and the strand transfer of M-PMV and HIV-1 long-terminal repeat-derived substrates with similar efficiency. The structure-based sequence alignment of M-PMV, HIV-1, SIV, and ASV integrases predicted critical amino acids and motifs of M-PMV integrase for metal binding, interaction with nucleic acids, dimerization, protein structure maintenance and function, as well as for binding of human immunodeficiency virus type 1 and Rous avian sarcoma virus integrase inhibitors 5-CI-TEP, DHPTPB and Y-3.