Four new macrocyclic polypeptides were isolated and identified from an extract of the tropical tree Chassalia parvifolia. Circulins C-F are 29-30 amino acid cyclic peptides in which the entire primary amino acid chain is covalently cyclized via peptide bonds. Their structures were deduced from a combination of FABMS analyses, N-terminal Edman degradation, endoproteinase digestion, and amino acid analyses. All the peptides share a high degree of sequence homology and contain six cysteine residues forming three intramolecular disulfide bridges. Circulins C-F inhibited the cytopathic effects of in vitro HIV-1 infection with EC(50) values of 50-275 nM.