The amido black and orange G methods for protein estimation in milk were investigated. The quantity of dye required for complete precipitation of a given quantity of milk protein was determined.

Dye samples of various degrees of purity all gave a linear relation between protein and dye precipitation, as determined by optical density, but with the less pure samples slightly less dye was bound per unit weight of protein.

Amido black, though reacting with protein in the same molar ratio as orange G, gave a much more sensitive optical indication of protein content.

In milks of individual cows there was a relatively constant relationship between dye values and Kjeldahl protein from 3 to 4 days after calving until 3 to 4 weeks before the end of lactation. Late lactation milks gave about 5% higher dye values than did normal milks of the same protein content.

The relation of dye precipitation to Kjeldahl nitrogen was the same for casein as for total nitrogen in normal milks.