Supernatant fractions (300,000 x g, 60 min) from homogenates of rat liver, heart, and skeletal muscle, dog liver, and rabbit liver prepared without detergent in the homogenization medium (referred to as S300) are shown to contain an activity that restores Mg2+-dependent fluoride- and guanine nucleotide-stimulated cyclizing activity to the adenylate cyclase system [ATP pyrophosphate-lyase (cyclizing); EC 4.6.1.1] in cyc- S49 murine lymphoma cell membranes. Approximately 25% of the total cyc- reconstituting activity in the above tissues is present in S300. Reconstituting activity is proportional to S300, is sensitive to trypsin, is protected against heat inactivation by guanine nucleotide, and has a sedimentation coefficient of 5.3 in both H2O and 2H2O linear sucrose density gradients. Treatment with cholera toxin and NAD+ results in reconstitution of cyc- adenylate cyclase with enhanced activity in the presence of GTP. Reconstituion with S300 is stable, as seen in cyc- membranes after washing. All of these properties of S300 are similar to those of membrane-derived cyc- reconstituting activity. It is concluded that cell cytoplasm contains a naturally soluble protein or mixture of proteins having guanine nucleotide regulatory component activity of adenylate cyclase.