It was investigated by MILLAR that tyrosine was readily converted into dibromtyrosine by the action of bromine, and that by means of this reaction tyrosine could be determined in mixtures of amides and aminoacids. PLIMMER and EAVES (1913) applied this reaction with some modification to the determination of tyrosine in proteins. Lately PLIMMER and PHILLIPS (1924) studied this reaction more closely, and applied it to the determination of tyrosine and histidine. A sample acidified with HCl was treated with some excess of potassium bromide and sodium bromate. And after bromination some potassium iodide was quickly added and the liberated iodine was titrated with standard sodium thiosulphate solution. They described that by bromination of the cleavage products of proteins, histidine can be estimated in the phosphptungstic acid precipitate and tyrosine in the filtrate, and that both histidine and tyrosine absorbe two atomes of bromine, respectively. It is obvious that PLIMMER and PHILLIPS' method cannot be used in the presence of cystine since OKUDA (1916), THRUN and TROWBRIDGE (1918) have already shown that one molecule of cystine reacts with IO atoms of bromine in acid solution. And PLIMMER and PHILLIPS also found that cystine reacts with bromine. But even in absence of cystine, I failed to justify the findings of PLIMMER and his coworkers (EAVES and PHILLIPS) about tyrosine and histidine. According to my experiments tyrosine and histidine react with more than two atoms of bromine, dependig on the temperature and time of bromination. Thus we may conclude that their methods for the determination of tyrosine and histidine are not satisfactry.