Several investigators have failed to confirm any specificity of elevated serum ribonuclease (RNase) in the diagnosis of pancreatic cancer. Although RNase had been known to be present in two forms, free and inhibitor-bound, in various tissues of the rat, little was known about it in the human pancreas. The object of this report was to explore the presence of RNase inhibitor in the human pancreas through the assay of both active (= free) and total (= sum of free and inhibitor-bound) RNases. Inhibitor-bound RNase is also referred to as latent RNase. RNase was classified into three types according to pH (acid, neutral, and alkaline RNases) in the pancreatic supernatant fraction. An inhibitor was separated from latent RNase by p-chloromercuribenzoic acid (PCMB), and the latent RNase was changed to an active form. Latent RNase was more active on the alkaline side with a maximum at pH 7.5. Hence, the presence of RNase inhibitor was highly probable in the pancreatic supernatant fraction. RNase inhibitor is most likely a protein, which is bound with both neutral and alkaline RNases. RNase inhibitor may be a cause of nonspecificity and/or low sensitivity of RNase in the serum as a diagnostic marker for pancreatic cancer.