Blocking human immunodeficiency virus (HIV) entry into target cells is an important goal of HIV and acquired immune deficiency syndrome (AIDS) therapies. We have searched for anti-HIV substances from microorganisms using a syncytium formation assay system constructed with HeLa/CD4/Lac-Z and HeLa/T-env/Tat cells, We discovered a novel anti-HIV protein that inhibits syncytium formation, designated as actinohivin, from a cultured broth of a soil isolate, actinomycete strain K97-0003. ESI mass spectrometry of actinohivin isolated from the culture filtrate showed an ion with molecular mass of 12,520.3 De. The amino acid sequence was determined by N-terminal Edman degradation of the intact protein and peptide fragments formed by endoproteinase digestions. Actinohivin consists of a 114-amino-acid chain that exhibits internal sequence triplication. Actinohivin inhibited both T-cell and macrophage tropic syncytium formation, with IC50 values of 60 and 700 nM, respectively, and the cytopathic effect of HIV-1(IIIB) in MT-4 cells, with IC50 value of 230 nM,