The widespread use of bovine serum albumin preparations for the stabilization of purified glycosyltransferases has prompted us to study the effects of different preparations of albumins on the galactosyltransferase activity of bovine milk. For comparison, several other proteins were tested as well. The albumins caused a large stimulation of transferase activity (400-700%) which varied depending on the source of the albumin and the treatment to which it had been subjected. Several other unrelated proteins were tested for their effects on transferase activity. Some proteins stimulated, while others had little effect. Lysozyme stimulated the activity by 178% and poly-L-lysine had little effect. Other proteins stimulated to variable extents. The stimulations obtained with albumin and myelin basic protein were noteworthy. The stimulation was considerably less marked when the enzyme was incorporated into lipid vesicles. These results emphasize the need for caution when adding proteins such as bovine serum albumin to purified enzymes for the purpose of stabilizing the activity of the enzyme.