Label incorporation into DNA in vivo

[Igor Sidorenkov]

Hello,
	I am trying to demonstrate inhibition of DNA synthesis in S.mu
tans by some chemical agents and have a following problem. When I add 
3H-thymidine only to growing culture (OD550=0.3) I have about 100% 
incorporation within 5-10min. However, if I dilute label by cold thymidine
(1mg/l final concentration) or resuspend washed cells in fresh media 
(35mg/l adenine), incorporation stops after 20-30 min at 10% of maximum 
level. What might be the reason?