Sera from normal volunteers, patients with a variety of non-neoplastic diseases and patients with malignant or benign tumors were examined by two-dimensional acrylamide gel electrophoresis. In this technique the serum is first separated in an acrylamide gel column followed by a second electrophoresis at right angles to the first separation in a continuous concave 2 to 30 percent gradient acrylamide gel slab. The stained two-dimensional gel slab appears as a "fingerprint" pattern or "map" of the separated serum proteins. Both qualitative and quantitative differences in the fingerprint patterns of cancer-patient sera were observed. The quantitative alterations did not appear specifically associated with malignant tumors. However, several qualitative differences were detected, of which some may represent markers of malignancy as they were not observed in the normal, abnormal and benign tumors control sera. At least one of the abnormal protein stained spots, a prealbumin, appears to be restricted and related in some way to cancer of the lymphoreticular system. These data, although limited, support earlier indications that two-dimensional acrylamide gel electrophoresis offers a new and powerful tool to the cancer scientist for the detection of alterations in cancer-patient sera. A discussion of its possible use to examine other biological fluid and tumor extracts from the cancer patient is presented.