Bioassay-directed fractionation of the extract of the cyanobacterium P. tenue led to the isolation of the three classes of glycolipids, viz., monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), and sulfoquinovosyl diacylglycerol (SQDG) as anti-tumor-promoters. In comparing the anti-tumor-promoting effect of the three classes of glycolipids with various acyl residues, MGDG and DGDG inhibited tumor promotion more intensely than SQDG. With respect to MGDG and DGDG, the inhibitory activity of each glycolipid possessing a single pair of acyl residues was also investigated after HPLC separation. Among the 17 tested MGDGs (1a--h) and DGDGs (2a--i), three of the DGDGs (2a, b, e) showed more potent inhibitory activity toward tumor promotion than the others.