An aqueous extract of green laver (Enteromorpha sp.) added to skim milk stimulated the acid produced by Lactobacillus casei YIT-9018 to as high a level as that of chlorella.<BR>The active component in the extract for stimulating this acid production was studied. The factor stimulating acid production was fractionated by gel filtration with G-50 and G-15, and by chromatography with Dowex 50 and Dowex 1. The fraction containing stimulating activity was separated from the sugar, amino acid and peptide fractions, as well as most of the nucleic acid derivatives.<BR>Since most of the stimulating activity still remained after ashing, and the level of activity was proportional to the amount of Mn in the ashed fraction, it was concluded that Mn was the component responsible for the stimulating effect on acid production of green laver. An organic substance, not Mn, seemed to be involved in the stimulating activity with L. casei subsp. casei S-1.<BR>The effect of the extract from sea lettuce (Ulva sp.) was to stimulate acid production even more than that by green laver, the Mn content of sea lettuce also being greater than that of green laver.<BR>Aqueous extracts from the two algae considerably stimulated the acid produced by L. delbrueckii subsp. bulgaricus B-5b, L. acidophilus L-54, L. casei subsp. casei L-14, L. paracasei subsp. paracasei IFO-3533, and L. paracasei subsp. paracasei S-2, but significantly inhibited it with L. helveticus B-1 and L. delbrueckii subsp. bulgaricus IFO-13953.<BR>However, neither extract had much effect on the acid production by Lactococcus lactis subsp. lactis 527, Lactococcus lactis subsp. cremoris H-61, or Streptococcus salivarius subsp. thermophilus 510.