The fat and protein contents of the foremilk and hindmilk from each milk duct differed despite being expressed from the same breast. When we compared the CrCt values and protein content obtained from three ducts, a greater than 5% difference between the maximum and minimum value was observed in 64 out of 68 breasts (94%) for CrCt and in 61 out of 68 breasts (91%) for protein content. The mean COV of protein content in each breast was 13.2 +/- 8.0 (range, 1.2-45.3).