Recently, naringinase preparations of molds such as Asp. niger and Asp. usamii mut. shirousamii were brought to market in Japan. Since the enzyme hydrolyses naringin, a bitter substance in fruits of Natsudaidai or grapefruit, the preparation is employed for removing bitterness in processing of the fruits. According to Okada et al. and Takiguchi, naringin (naringenin-7-rhamnoglucoside) is hydrolysed by releasing rhamnose and glucose as shown in Scheme 1. Therefore, it might be understood that naringinase action is retarded by rhamnose or grucose, as established by several authors. Natsudaidai contains sucrose and fructose as well as glucose. In addition, sugar was often supplemented in processing of the fruits to sweetening the products. Therefore, in order to obtain helpful data to employment of the enzyme preparation, the influence of several kinds of sugar on naringinase was investigated. Enzymatic activity was assayed by determining hydrolysis of naringin which was colorimetrically estimated by Davis' method using diethyleneglycol and sodium hydroxide. Naringinase was strongly inhibited by rhamnose or glucose of low concentration. Inhibiting rate of rhamnose was more or less higher than that of glucose. With galactose or mannose, hydrolysis of naringin was also retarded, even with less extent than with rhamnose or glucose. In this case, galactose has a little higher ability to inhibit the enzyme than mannose has. By xylose too, naringinase was inhibited in about the same extent with mannose. Similar degree of inhibition of naringinase was observed with fructose and sucrose which are natively contained in the fruits. Sorbitol has lesser ability to reduce naringinase activity, while least inhibition was estimated with ascorbic acid. Inhibition of naringinase by a mixture of sucrose and sorbitol was also estimated in order to examine whether or not sorbitol is useful to substitution for sucrose accompanying lesser inhibiting rate. These influence could be compared by consulting the inhibiting rates in Tables and the slope of straight lines of V/Vi plotted to concentration of sugars. When glucose had been transformed into glucosoxime, enhancement of inhibiting rate of glucose on naringinase was estimated. On the contrary, inhibition was reduced with glucosamine to the level by galactose. No alteration in inhibiting rates, however, was established, if galactos- or mannosoxime was substituted for galactose or mannose. Carbonyl reagents such as hydroxylamine and hydrazine had no effect on naringinase, whose activity was reduced by SH reagents, monoiodoacetic acid and PCMB, of low concentration. Incidentally, some inhibition with α-ketoglutaric acid was observed, but not with acetaldehyde. By discussing these results, it was suggested in particular that glucose content in Natsudaidai, about 1.6~1.8%, has a significant effect on naringinase to inhibit the activity, whereas contents of fructose and sucrose, about 1.1~1.3 and 2.7~3.1%, have negligible influence.