TWO new and potent antimitotic metabolites, curacins B 127 and C 137, were isolated from a Cuqao collection of Lyngba mjuscula. The structures of 2 and 3, geometrical isomers of the known compound curacin A Ill, were determined by detailed spectroscopic analysis in comparison with 1. The absolute stereochemical configuration of 2 was deduced to be 2R,13R,19R,21S by its thermally induced interconversion with 1. Different isolates of the marine cyanobacterium Lyngbya majuscula (Oscillatoriaceae) produce an amazingly wide array of biologically active secondary metabolites, reminiscent of the rich biosynthetic capacity of some species of Streptomyces. The potent tumor promoters lyngbyatoxin and debromoaplysiatoxin (1,2) and the antifungal lipopeptides majusculamides A, B, and C (3,4) are examples of the range of metabolites isolated from this organism. Recently, our examination of a Curacao (Caribbean) collection of Lyngbya mujuscula has resulted in the isolation of an extremely potent antimitotic agent, curacin A El] (5,6), which is under examination for its potential anticancer utility (7,8). In continuing investigations of this organism for minor metabolites of related structure, we have isolated and characterized two new natural products, curacins B 127 and C C37. Both of these are toxic to brineshrimp, demonstrate strong cytotoxicity against murine L-1210 leukemia and human CA46 Burkitt lymphoma cell lines (7,8), inhibit the polymerization of purified tubulin in vitro, and in the NCI in vitro 60-cell line assay, show potent antiproliferative activity to many cancerderived cell lines in a manner characteristic of antimitotic agents.