| Summary: | <i>Broussonetia kazinoki</i> has been used as a traditional medicine for the treatment of burns and acne, and its extracts have been found to show tyrosinase inhibitory and anticancer activities. In this study, the tyrosinase inhibitory and cytotoxic activities of <i>B. kazinoki</i> were explored, leading to the isolation of kazinol C (<b>1</b>), kazinol E (<b>2</b>), kazinol F (<b>3</b>), broussonol N (<b>4</b>), and kazinol X (<b>5</b>), of which the compounds <b>4</b> and <b>5</b> have not been previously reported. Microbial transformation has been recognized as an efficient tool to generate more active metabolites. Microbial transformation of the major compounds <b>1</b> and <b>3</b> was conducted with <i>Mucor hiemalis</i>, where four glucosylated metabolites (<b>6</b>–<b>9</b>) were produced from <b>1</b>, while one hydroxylated (<b>10</b>) and one glucosylated (<b>11</b>) metabolites were obtained from <b>3</b>. Structures of the isolated metabolites were determined by extensive spectroscopic analyses. All compounds were evaluated for their tyrosinase inhibitory and cytotoxic activities. Compound <b>3</b> and its metabolites, kazinol Y (<b>10</b>) and kazinol F-4″-<i>O</i>-β-<span style="font-variant: small-caps;">d</span>-glucopyranoside (<b>11</b>), exhibited the most potent tyrosinase inhibitory activities with the IC<sub>50</sub> values ranging from 0.71 to 3.36 µM. Meanwhile, none of the metabolites, except for kazinol C-2′,3″-di-<i>O</i>-β-<span style="font-variant: small-caps;">d</span>-glucopyranoside (<b>7</b>), showed moderate cytotoxic activities (IC<sub>50</sub> 17.80 to 24.22 µM) against A375P, B16F10 and B16F1 cell lines.
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