In the process of isolating sialic acid from bovine isoelectric casein a hydrolytic product (CBF) was obtained, possessing growth promoting activity for Bifidobacterium bifidum var. pennsylvanicus. Growth was measured turbidimetrically by addition of CBF (250 ppm) to ATCC Medium 76. Stimulation by CBF was comparable to that obtained with N-acetylglucosamine. Whole casein, sialic acid-free casein and k-casein failed -to stimulate the growth of -the organism to the level achieved with CBF. The CBFfraction contained 0.15% nhosohorus. 7% free lactose and various peptides. No sialic acid or ofher amino sugars were detected. On the basis of phosphorus content and amino acid analysis it was concluded that CBF was likely a degradation product of k-casein.