The cellular viability of the human keratinocyte cell line HaCaT was compared after adding seaweed extracts to the culture medium. The viability was measured using a quick, quantitative, spectrophotometric crystal violet inclusion method. Of 36 common seaweed species tested, methanol extracts from Sargassum sagamianum and Gigartina tenella enhanced the viability of HaCaT cells by 1.6-fold, as compared to control cells, while methanol extracts from Dictyota dichotoma, Pachymeniopsis elliptica, and Enteromorpha linza decreased the viability to less than half that of controls.