The effect of tracer doses of 125IUdR on the biological properties of tumor cells was studied.Experimental results demonstrated that the micronucleus rates of B16 melanoma cells increased significantly when the dose of 125IUdR was 0.1934 dpm/cell-DNA. The doubling time of cells obviously increased and the regression coefficient of in vitro cell growth curve decreased significantly at 1.3323 dpm/cell-DNA. Lung metastatic colonies of B16 melanoma decreased significantly when the dose of 125IUdR was 1.1795 dpm/cell-DNA. The subcutaneous growth of tumor cells have been inhibited obviously when the dose was 0.1934 or 0.3079 dpm/cell-DNA. Therefore

the dose of 125IUdR in tracer experiments should be carefully controlled

preferably below 0.2 dpm/cell-DNA.