Protein kinase that phosphorylated histone and lesser amounts of protamine was demonstrated in human heart. It was activated three times by 10(-6) M cyclic adenosine 3':5'-monophosphate (cAMP) and by 10(-3) M other cyclic nucleotides. Km values for cAMP, ATP, Mg2+, and Co2+ were about 2 X 10(-8) M, 4 X 10(-5) M, 2 X 10(-3)M, and 1.7 X 10(-4) M, respectively. On DEAE cellulose column, the main peak of the enzyme eluted at high NaCl concentration. On Sephadex G-200 gel filtration the majority of the holoenzyme eluted at a peak corresponding to a molecular weight of about 300,000. There was an additional peak corresponding to a molecular weight of about 400,000, with relatively high cAMP binding compared to kinase activity. Right atrium and ventricle showed significantly higher enzyme activities than left atrium and ventricle and interventricular septum. On multivariate analysis of the enzyme activity versus 12 clinical and pathological findings of 122 cases, cardiac hypertrophy and coronary sclerosis were slight but significant negative contributors to the enzyme activity. Multiple correlation coefficient was low, indicating the enzyme activity remained at a relatively stable level, despite different clinical situations. This may be suitable for control of intracellular events through the membrane adenylate cyclase system.