DURING the course of experiments conducted on the sheep foetus, dyes were injected into the umbilical cord with the object of exploring movements of substances in the cord along non-vascular pathways. The dyes used were a dis-azo dye, related to trypan blue, and the same dye linked to serum albumin by a diazo linkage. The dye, when linked to protein in this way, cannot dissociate from the albumin, and hence acts as a tag to the protein. These substances were prepared by one of us (P.D.M.).