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Anti-human immunodeficiency virus (HIV)-bioassay-guided fractionation of aqueous extracts of the Caribbean sponge Niphates erecta led to isolation of a novel anti-HIV protein, named niphatevirin. The protein was purified to homogeneity by ethanol precipitation, ammonium sulfate precipitation, gel-permeation chromatography and concanavalin-A—Sepharose affinity chromatography. Niphatevifin potently inhibited the cytopathic effects of HIV-1 infection in cultured human lymphoblastoid (CEM-SS) cells; the effective concentration of drug that results in 50% protection of the cells through inhibition of ceil lethality, cell-cell fusion and syncytium formation was approximately 10 nM. Delay of addition of niphatevirin to infected cultures by two hours markedly decreased (≈50%) cytoprotection; delay of addition by eight hours resulted in no antiviral activity. Niphatevirin bound to CD4 in a manner that prevented the binding of gp120, but did not directly bind gp120. Niphatevifin (6.5 μM) was inactive in both hemagglutination and hemolysis assays. Niphatevirin had a molecular mass of about 19 kDa by matrix-assisted laser-desorption ionization-time of flight (MALDI-TOF) mass spectrometry, and a native molecular mass of approximately 18 kDa by gel-fltration chromatography. The protein had an acidic isoelectric point of 4.2–4.6, and was shown by periodate acid Schiff's staining to be glycosylated. [ABSTRACT FROM AUTHOR]