Previous investigations 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 rat tissues, little was known about its presence in the human pancreas. This report investigates 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 was also named 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 suggested in the pancreatic supernatant fraction. RNase inhibitor is most likely a protein, which binds with both neutral and alkaline RNases. The presence of RNase inhibitor has not yet been demonstrated in the normal human serum.