AbstractA membrane‐bound and a soluble cyclic nucleotide phosphodiesterase from sunflower callus has been examined. Kinetic studies showedKm‐values in the range of 2.5‐5.0 mMwith cyclic adenosine 3′,5′‐monophosphate (3′, 5′‐cAMP) as the substrate. The soluble cyclic nucleotide phosphodiesterase was characterized further: Optimal activity was at pH 6.5‐7.0 and at 40°C. The enzyme did not require divalent metal ions for maximal activity. It was inhibited by Cu2+and slightly activated by Fe3+. Sodium fluoride, imidazole and theophylline did not affect the activity, but caffeine was slightly inhibitory. Its activity was strongly inhibited by ATP, AMP and Pi. The enzyme hydrolysed 3′,5′‐cAMP, 2′,3′‐cAMP and 3′,5′‐cGMP, while N6, O2′‐dibutyryl cyclic AMP (dBcAMP) was left unattacked. The soluble enzyme activity was influenced neither by callus age nor by plan