Porphyrans, the sulfated polysaccharides, are the main components of Porphyra. The potential apoptotic activities of porphyran were evaluated using AGS human gastric cancer cells. Porphyran did not affect the growth of normal cells, but did induce cancer cell death in a dose-dependent manner. The addition of 0.1% porphyran also reduced DNA synthesis after 24 h of exposure, suggesting that porphyran inhibits cancer cell growth by both decreasing cell proliferation and inducing apoptosis. AGS cells treated with porphyran displayed a marked increase in poly(ADP-ribose) polymerase (PARP) cleavage, as well as caspase-3 activation. The ability of porphyran to promote apoptosis may contribute to its usefulness as an agent capable of significantly inhibiting cell growth in AGS human gastric cancer cells. Insulin-like growth factor-I receptor (IGF-IR) phosphorylation was decreased in porphyran-treated AGS cells compared to control cells, which correlated with Akt activation. Thus, porphyran appears to negatively regulate IGF-IR phosphorylation by causing a decrease in the expression levels in AGS gastric cancer cells, and then inducing caspase-3 activation.