The kinetic properties of cyclic nucleotide phosphodiesterase isolated from the cytoplasmic fraction of lymphoblastoma QOS cells were studied. It was demonstrated that the enzyme can be activated in the presence of micromolar concentrations of cGMP. The kinetic properties of the enzyme are characterized by the nonlinear dependence of the cAMP hydrolysis rate on the substrate concentration. The curve becomes linear in the presence of cGMP. The molecular mass of phosphodiesterase as determined from gel filtration data is 80,0000 Da.