Abstract
The addition of serum to density‐inhibited human fibroblast cultures induced a wave of DNA synthesis, measured as [^3^H] thymidine incorporation into acid‐precipitable material, beginning after 8–12 hr and reaching maximum levels at 16–24 hr. Addition of dibutyryl‐3′ : 5′‐cyclic AMP (DBcAMP) together with serum inhibited [^3^H] thymidine incorporation by 75–95%. When DBcAMP was added for the first 4 hr of serum stimulation and then removed, the wave of DNA synthesis was not delayed. This suggested that serum could induce DNA synthesis even though cyclic AMP concentrations were maintained at high levels by DBcAMP during this initial period. These results are inconsistent with the hypothesis that it is the immediate transient reduction in 3′ : 5′‐cyclic AMP concentration following the addition of serum that triggers DNA synthesis. By contrast, DBcAMP added 8 hr after serum inhibited [^3^H] thymidine incorporation to the same extent as DBcAMP added at the same time as serum. This indicated that a step essential for DNA synthesis and occurring late in G~1~ was inhibited by high concentrations of 3′ : 5′‐cyclic AMP.