A highly sensitive detection system for acid proteinase separated on polyacrylamide gel was established. This system consisted of two-dimensional electrophoresis, combined with isoelectric focusing and polyacrylamide gel electrophoresis, and casein clotting (caseogram). Human urine, serum and gastric tissues obtained from normal individuals and gastric cancer patients were analyzed using this system. The previous electrophoretic method was not sufficiently sensitive to detected small amounts of pepsinogen (PG) C in normal urine. However, the new rapid and sensitive method clearly revealed its presence. In gastric tissue containing cancer cells, an additional proteinase, which was not present in normal tissue, was detected and named medium moving proteinase (MMP). MMP resembled PGs in alkaline stability rather than the non-PG proteinase, slow moving proteinase (SMP).