A collection of Lyngbya majuscula from Palau contained the peptides dolastatin 3 (1), homodolastatin 3 (2), and kororamide (3), together with aplysiatoxin (4), debromoaplysiatoxin (5), and oscillatoxin A (6). The structures of the new peptides homodolastatin 3 (2) and kororamide (3) were determined by interpretation of spectroscopic data and chemical degradation.As part of a collaborative effort to discover new inhibitors of HIV-1 integrase from marine organisms, 1 we screened a collection of crude extracts of marine organisms from Palau. Among the few extracts that showed activity, most of which were from marine sponges, was an extract of the macroscopic cyanophyte Lyngbya majuscula that had been collected because it was fouling corals. L. majuscula has previously yielded a wide variety of natural products 2 that include the antiproliferative agents, curacin A 3 and debromoaplysiatoxin, 4 and both acyclic 5 and cyclic 6 peptides. In this paper we report the HIV-1 integrase inhibition assayguided isolation of dolastatin 3 (1), which had previously been reported from the sea hare Dolabella auricularia, 7 and the structure elucidation of two new cyclic peptides, homodolastatin 3 (2) and kororamide (3).The specimen of L. majuscula (90-95% homogeneous) was collected in shallow water in the lagoon near Big Goby marine lake, Palau, and was immediately frozen. The crude methanolic extract of the cyanophyte was found to inhibit HIV-1 integrase in the terminal-cleavage and strandtransfer assays, 8 but did not inhibit molluscum contagiosum virus (MCV) topoisomerase. 9 The ethyl acetate-soluble material from the methanolic extract was subjected to repeated chromatography on reversed-phase supports to obtain dolastatin 3 (1, 4 × 10 -5 % wet wt), homodolastatin 3 (2, 4.8 × 10 -5 % wet wt), and kororamide (3, 4 × 10 -5 % wet wt), in addition to the known metabolites aplysiatoxin (4), debromoaplysiatoxin (5), and oscillotoxin (6). 4,10 Each fraction obtained during the separation procedure was screened for activity against HIV-1 integrase, and it was shown that the only active material present was dolastatin 3 (1), which was identified by comparison of its spectral data with literature values. 7 Homodolastatin 3 (2) was obtained as a white solid. The molecular formula, C 30 H 42 N 8 O 6 S 2 , obtained from a highresolution mass measurement of the [M + Na] + ion, suggested that 2 differed from dolastatin 3 (1) by the addition of a methylene group. Analysis of the 1 H and 13 C NMR spectra revealed signals that were characteristic of an isoleucine residue in place of the valine residue of dolastatin 3. The sequence of the amino acid residues was confirmed by analysis of the HMBC experiment. The absolute configuration of the L-isoleucine, L-proline, and L-leucine residues was determined by hydrolysis followed by GC-MS analysis of the derivatized residues on a chiral column and that of the remaining center was determined by treatment with ozone followed by hydrolysis and derivatization to obtain a peak corresponding to L-glutamate,...