In serially independent samples of blood from apparently healthy subjects controlled as to clock-hour of blood withdrawal but not as to any circannual changes in circadian state, human erythrocyte adenosine aminohydrolase undergoes a statistically significant circannual rhythm, which may or may not be aliased. This rhythm is also demonstrated in patients with a variety of neoplastic diseases and it occurs around a statistically significantly lower mesor in patients with a variety of malignant tumors investigated.