Novel potent cytotoxic peptides bisebromoamide (1) and norbisebromoamide (2) have been isolated from the marine cyanobacterium Lyngbya sp. The planar structure of these peptides was elucidated through the extensive application of 1D and 2D NMR techniques. The absolute stereostructure of 1 was determined by chemical degradation followed by chiral HPLC analysis. Recently, Tao and co-workers achieved synthesis of bisebromoamide, and the configuration of thiazoline moiety was revised. We reinvestigated the stereochemistry of thiazoline moiety of 1. The structureeactivity relationships of bisebromoamide (1) were investigated with the use of natural and synthetic analogs. Furthermore, bisebromoamide (1) potently inhibited protein kinase: the phosphorylation of ERK in NRK cells by PDGFstimulation was selectively inhibited by treatment with 10e0.1 mM of 1.