The FDA recommends and the California Agricultural Code requires that the milk supply be continuously screened for somatic cell number. Milk with counts over 1.5 million cells per milliliter is not acceptable. The number of somatic cells in milk from individual cows is an indication of udder health. A somatic cell count in the range of 30,000 to 300,000 per milliliter is currently considered normal; a count of 500,000 or more per milliliter is indicative of mastitis or other udder abnormalities.

John Bruhn, Agriculturist with Cooperative Extension, reviewed and edited the manuscript. Dairymen s Cooperative Creamery Association, Tulare, California, donated the use of their laboratory and their Coulter Counter. W. G. Whittlestone, Ruakura Research Station, New Zealand, donated the Ruakura Rolling Ball Viscometer for use in California.