Inhibition of M-Ras–induced proliferation of BaF3 by MAP kinase inhibitor. BaF3 cells expressing constitutively activated M-Ras/71K (3,000/well) were incubated with increasing concentrations of PD98059 in the presence or absence of 200 U/mL of IL-3. Thymidine incorporation was measured after 3 days. The results are presented as the percentage of the proliferation without PD98059 (means of triplicate wells). The same results were obtained with other activated M-Ras mutants (data not shown).