The measurement of protein with copper and the Folin reagent has cer- tain advantages. (1) It is as sensitive as with Nessler’s reagent, yet requires no digestion. (2) It is 10 or 20 times more sensitive than measurement of the ultraviolet absorption at X = 280 rnp and is much more specific and much less liable to disturbance by turbidities. (3) It is several fold more sensitive than the ninhydrin reaction (23) and is some what simpler, as well as much easier to adapt for small scale analyses. Free amino acids give much more color than proteins with the ninhydrin reaction, whereas the reverse is true with the Folin reagent. (4) It is 100 times more sensitive than the biuret reaction.