Human lymphocytes were treated with varying radioactivities of

3

H-TdR and

3

H-UdR in vitro and using the fluorescence plus Giemsa (FPG) technique to estimate the effects of tritium on the cell cycle. The results indicated that two kinds of tritiated compounds delayed the cell cycle of the proliferating lumphocytes in culture. The effects of delaying cell cycle were markedly increased with increasing the radioactivities of tritium. According to the efficiency of delaying cell cycle

when the radioactivity/ml of the medium was as high as 3.7×10

4

Bq/ml

two compounds could be arranged in decreasing order as follows:

3

H-UdR

>

3

H-TdR. The amount of tritium incorporated into cells could be affected by BrdU in the medium. Absorbed doses were increased with increasing time of culture. From the absorbed doses it could be seen that the different extent of delaying cell cycle between the two tritiated compounds could be due to different amount of tritium incorporated into cells. The results demonstrated that the tritiated compounds could affect lymphocyte proliferating rate. The cell proliferating rate index and mitotic index were decreased with increasing the absorbed dose.