It was proved that tea-seed saponins (TSS) showed the growth inhibitory activity to the weed seedlings, early watergrass (Panicum Crus-galli L.), green foxtail (Setaria viridis Beauv. L.) and white clover (Trifolium repens L.) at concentrations more than 10 mg/L. However, this activity was lower than agricultural chemicals. The additive growth inhibitory effect of TSS and glyphosate (isopropylammonium N-(phosphonomethyl)glycinate), which is one of the most popular weed killers in Japan, was investigated to prove that TSS additively increased the growth inhibitory effect of glyphosate to early watergrass seedlings. This result suggests that TSS could be utilized as growth inhibitory agents of weed seedlings in combination with other agricultural chemicals.