The series of analyses and deductions to be reported were made on one hundred myofibromata removed during operation. The objective of the work was (a) the determination of the variability of the potassium content in this easily procurable form of neoplasm and (b) the confirmation or refutation of the belief that the potassium content of tumors is greatly increased above that of normal tissue of essentially the same category. As the work progressed, a third objective developed: i.e., a possible correlation between the amount of potassium and the macroscopic appearance of the tumors. A classification according to microscopic diagnosis would have been far more accurate, but this was not possible. As will be seen, the first two objectives were fairly well attained, the third less completely so. Method With but little alteration the methods used for determining potassium, ash, and water in the tumors were the same as those already reported by the author in determining these constituents of heart muscle (1). The differences lay in using only 50 gm. of fresh tissue, not removing the phosphates by a special precipitation, and washing the reduced platinum with hot dilute hydrochloric acid. A number of comparative determinations had shown that the phosphates did not interfere with the final result and that about 25 c.c. of approximately 10 per cent hydrochloric acid was quite sufficient to remove all insoluble phosphates. Numerous checks were made in duplicate and triplicate, with and without precipitation of phosphates. These were satisfactory.