When 5-Chloro-2’-deoxyuridine (ClDU) is placed into tissue culture medium, mammalian cells incorporate the analog into DNA. It is observed that 10 μM concentration of 5-Chloro-2’-deoxyuridine in the media does not alter cell division kinetics. Previously it has been shown that 5-Chloro-2’-deoxyuridine is metabolized and incorporated into DNA using antibodies that bind selectively to DNA containing halogenated bases. In the studies reported here, 5-Chloro-2’-deoxyuridine is more similar to BrdU in acting as a T analog. The toxicity of 5-Chloro-2’-deoxyuridine could in part be attributed to inhibition of thymidylate synthase.