Addition of the chelator DTPA to synchronized cultures of 3T3 cells inhibited thymidine incorporation by up to 90% and only Zn2+ of the divalent cations tested was effective in reversing this effect. Fe2+ given alone had no effect on the inhibition by DTPA but when added to Zn2+ supplemented cultures increased thymidine incorporation from approximately 80-90 to over 100% of that in control cultures. Investigations indicated that the major requirement for Zn2+ was within the period from 8 h after stimulation of quiescent cells with serum until 3 h before the start of S phase. There was also an indication of a further requirement for Zn2+ at the G/S transition.