In vitro selective antitumor activity was tested as a general screening parameter for biologically active substances from a wide range of species of seaweed, 1446 samples of 306 species of marine algae from Japan's coasts. The algae extracts were prepared successively first by phosphate buffered saline (PBS) and then by methanol, and then tested for in vitro selective antitumor activity against murine lymphoid leukemia L1210 cells and for low cytotoxic activity against NIH-3T3 normal cells.Strong cytotoxic activity against L1210 cells was found in 47 species of algae, also showing similar cytotoxicity to mouse NIH-3T3 normal cells. However, four species of green algae showed strong activity specifically against L1210 cells, with low cytotoxicity to normal cells. Such selective activity was conspicuous in two brown and two green algae extracts. In particular, methanol extracts from the green alga, Cladophoropsis vaucheriaeformis, exhibited high viability (86%) to normal cells, showing selective cytotoxicity to tumor cells. This alga extract was not cytocidalic, but cytostatic against L1210 cells. Furthermore, the results of a cytotoxic spectrum test with 9 cell lines including those of L1210 and NIH-3T3 demonstrated that this extract acted strongly only against leukemic cell lines L1210 and P388.