Fig. 3. Effect of EGF and FBS on FRT and HeLa cell proliferation. Serum-arrested cells were incubated with different concentrations of EGF for 1 hour (A) or with 10% FBS for either 1 hour before, or 16 hours during [³H]thymidine incubation (B). Only FRT cells show a proliferative response to EGF, indicating concentration dependency. None of the cells responded to just 1 hour of FBS, contrasting with the effect of a long incubation period, which could override the proliferative arrest (n=4-6).