1. Buscaino's reaction was studied chiefly in psychoses and for purposes of control in diseases of other systems and in normal individuals.2. In the group of nervous and mental diseases of the total number of 147 patients, 43, or 29.25 per cent, showed a positive reaction. The latter was most frequently positive in anergastic disorders (organic reaction types) namely, in 47.5 per cent of our 40 cases. In the groups of thymergastic (manic-depressive), merergastic (psychoneuroses), parergastic reaction types (schizophrenia, paranoia), epilepsy and some unclassified cases, it was positive in a percentage between 11.1. and 33.3 per cent.3. In the control cases we obtained a positive reaction in 42.5 per cent of our 40 cases of acute and chronic affections of different systems and in 8.6 per cent in the group of 35 normal individuals.4. In the specimens of urines with positive black reaction the ratio between uric and sodium chloride was higher than in the urines with negative reaction.5. Both uric acid and sodium chloride were found in lower amounts in the positive cases than in the negative ones; and the high ratio between these two chemical compounds in the urines with black deposit, as it was noted, is essentially due to a low concentration of sodium chloride.6. In the light of the fact that, on the one hand the black reaction was found positive in varying pathologic conditions and even in a few normal individuals, and on the other hand that the positive reaction was accompanied in most cases by a higher ratio between uric acid and sodium chloride than the negative reaction, we are justified in concluding that the Buscaino reaction does not depend upon the diseases themselves, but only upon epiphenomena (such as changes in the relation between uric acid and sodium chloride). These changes are very likely, at least partly if not altogether, due to the diet of hospital patients which is usually reduced in proteins and salt. Our findings are thus completely unfavorable as to the value of the reaction.