Bioassay-guided fractionation was applied to the cytotoxic chloroform fraction of the red alga Polysiphonia lanosa. The major compounds of the most active fraction were identified using GLC-MS analysis as lanosol (1), methyl, ethyl, and n-propyl ethers of lanosol (1a, 1b, and 1c, respectively), and aldehyde of lanosol (2), although 1b appears to be an artifact arising during the fractionation procedure. These compounds and other known bromophenols were synthesized in addition to four novel isomers (3, 3a−c). The cytotoxic activities of all the synthetic compounds were determined against DLD-1 cells using the MTT assay. Compounds with IC50 < 20 μmol were also tested against HCT-116 cells. Compound 3c (2,5-dibromo-3,4-dihydroxybenzyl n-propyl ether) was the most active compound against both cell lines (IC50 = 1.72 and 0.80 μmol, respectively), and its effect on the cell cycle was studied using flow cytometry.