BACKGROUND: Formation of an HIV-1 particle with a conical core structure is a prerequisite for the subsequent infectivity of the virus particle. We have previously described that glycineamide (G-NH2) when added to the culture medium of infected cells induces non-infectious HIV-1 particles with aberrant core structures. RESULTS: Here we demonstrate that it is not G-NH2 itself but a metabolite thereof that displays antiviral activity. We show that conversion of G-NH2 to its antiviral metabolite is catalyzed by an enzyme present in bovine and porcine but surprisingly not in human serum. Structure determination by NMR suggested that the active G-NH2 metabolite was alpha-hydroxy-glycineamide (alpha-HGA). Chemically synthesized alpha-HGA inhibited HIV-1 replication to the same degree as G-NH2, unlike a number of other synthesized analogues of G-NH2 which had no effect on HIV-1 replication. Comparisons by capillary electrophoresis and HPLC of the metabolite with the chemically synthesized alpha-HGA further confirmed that the antiviral G-NH2-metabolite indeed was alpha-HGA. CONCLUSION: alpha-HGA has an unusually simple structure and a novel mechanism of antiviral action. Thus, alpha-HGA could be a lead for new antiviral substances belonging to a new class of anti-HIV drugs, i.e. capsid assembly inhibitors.