We prepared monoclonal antibodies specific for smg p21A, one of the low molecular weight GTP-binding proteins and possibly a suppressor molecule for ras p21. Two monoclonal antibodies (T22 and T212) reacted with smg p21A but not with Ki-ras p21, both of which were produced by Escherichia coli. These two clones detected an Mr 21,000 band in one-dimensional immunoblotting of extracts of a human pancreatic cancer cell line which was indistinguishable from a band detected by RASK-3, a monoclonal antibody specific for ras p21. However, T22 and T212 detected a single spot in two-dimensional immunoblotting that was clearly different from the three spots detected in the same cellular extracts by RASK-3. A series of normal and malignant human tissues were examined for the expression of smg p21A and ras p21 by immunohistochemical methods utilizing T22 and RASK-3. In essentially all tissues examined, both normal and malignant, smg p21A and ras p21 were expressed with great similarity. Expression of both molecules in all malignant tissues examined was coincident with that in normal tissues except that gastric cancer showed increased expression of the two molecules in comparison with normal gastric tissue.