The study was designed to investigate the toxic effects of Rhizoma Zedoariae (RZ) and nickel sulfide, and the effect of RZ on nickel sulfide induced unscheduled DNA synthesis (UDS) of human lymphocytes using an UDS assay of human lymphocytes. The results showed that TZ did not induce UDS, it could inhibit 3H-TdR incorporation with intracellular DNA when its' dose was 1 g/ml, that nickel sulfide and ultraviolet could largely induce UDS, that might be inhibited by RZ with a dose dependent relationship. These suggested that nickel sulfide induced damage of DNA might be inhibited by RZ, RZ was not mutagenic and DNA synthesis might be inhibited when the RZ dose amounted to 1 g/ml.