cAMP elevations inhibit CSF-1– and IL-3–stimulated mitogenesis in 32D/CSF-1R myeloid cells. Exponentially growing cells were seeded into RPMI, 10% FBS with either 5 nmol/L CSF-1 or 100 U/mL of murine recombinant IL-3 and the indicated doses of btcAMP (left panel) or forskolin (right panel). Forty-eight hours later, mitogenesis was determined by the colorimetric assay as described in Materials and Methods. Results are shown as a percentage of the absorbance at 490 nm obtained for cells growing in IL-3 and represent the mean ± SE of duplicate samples.