The micronucleus rates have been used as a biological end point

the relative biological effectiveness (RBE) of

125

IUdR compared with

3

HTdR were worked out on the basis of regression equations of dose-response calculated in this study. The results demonstrate that the total micronucleus rates and the micronucleus rates of cells increased with the increase of the rates of

125

IUdR uptake respectively

the regressed equations were y

1

= 0.895+0.029 x

y

2

=0.0776 +0.021 x. Similar results were also observed when

3

HTdR was used

its regressed equations were y

1

= 0.082+0.0039 x

y

2

=0.068+0.003 x. The RBE of

125

IUdR were 7.4 and 7 as compared with

3

HTdR.

These suggest that the radiotoxicity of

125

IUdR is much more marked than that of

3

HTdR. The advantages and disadvantages of

125

IUdR used as trcer compared with those of

3

HTdR were discussed finally.