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A compound stimulatory to the growth of S. thermophilus CH-1 was isolated from the cell-free filtrate of L.helveticus YM-1 in milk medium. The stimulant was identified as a peptide with a molecular weight of approximately 5000 and exhibited positive ninhydrin reaction. Some kinds of amino acids confirmed as aspartic acid, alanine, valine, glutamic acid, lysine, proline, leucine were rich in the stimulatory peptide hydrolysate. Among them, glutamic acid was most abundant. Judging from bioautographic results, glutamic acid and phenylalanine were expected to exert an important role for the stimulation.
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