1. Many immunologically related components occur in bovine serum and colostrum, but certain components in each are absent in the other (specific components). Two components (or groups) occur in colostrum which do not appear in serum, and these can be associated with electrophoretically slow and fast colostrum components.

2. Pre-colostral and adult urines contain low concentrations (<0·2 g./ 100 ml.) of protein which, though immunologically related to serum proteins, does not closely resemble any major serum component in physical properties.

3. Urines collected during the period of proteinuria contain increased concentrations of protein (up to 2 g./ 100 ml.) which is immunologically related to both serum and colostrum. From the presence of material related to specific colostrum components (and the absence of specific serum components), the added protein evidently originates from colostrum rather than serum. The total added protein possesses components with electrophoretic mobilities covering a wide range, with appreciable protein faster than albumin at pH 8. From sedimentation studies, the added proteins resemble the slower sedimenting colostrum proteins (β-lactoglobulin and lactalbumin) and contain no observable material with the sedimentation behaviour of immune lactoglobulin.

4. Immunologically, the added protein is related only to the electrophoretically faster of the specific colostrum components, and this applies also to post-colostral lymph in spite of its very high content of immune lactoglobulin. It follows that the specific electrophoretically slower colostrum components represent only a small proportion of the total immune lactoglobulin fraction.

5. The presence in urine, collected during the period of proteinuria, of antibody activity (deriving originally from the calf's mother), and of protein with the approximate electrophoretic mobility of, but without the sedimentation behaviour of γ-globulin, is to be explained probably on the basis of molecular degradation at some stage between absorption from the intestine and excretion in the urine.

6. The low molecular weight colostrum components, though present in low concentration in the lymph, are absent or below the detectable threshold in post-colostral sera (containing much material derived from immune lactoglobulin), and this is thought to be partly due to selective action by the kidney, allowing the passage of, and causing concentration of low molecular weight protein in the urine, whilst retaining higher molecular weight protein in the serum.

7. The results show that the proteinuria of the new-born calf arose mainly from low molecular weight protein in the colostrum which was absorbed from the gut together with the immune lactoglobulin. Owing to the small molecular weight, this, the former protein, was subsequently cleared from the circulation probably by glomerular filtration, accounting for the difficulty of detecting them electrophoretically and ultracentrifugally. On the other hand, the immune lactoglobulin with its much higher molecular weight was not so cleared from the circulation and is readily detected there.

The authors wish to thank Sir Alan Drury, F.R.S., for his support during the course of this work, Mr N. Buttress and Mr D. Hardman for technical assistance and Mr J. Clark for assistance in the care and handling of the calves.