A non-ionic detergent-resistant complex of membrane-associated proteins and cell-surface glycoproteins has been isolated by gel filtration and isopyknic centrifugation of purified plasma membranes from the murine T-lymphoma P 1798. This complex elutes as a high molecular weight peak (greater than 15 X 10(6) D) and contains two specific sets of (1) cell-surface glycoproteins; (2) membrane-associated proteins. The cell-surface glycoproteins consist of two vectorially labelled major components present in a fixed molar ratio: The Thy-1 glycoprotein and a non-H-2 glycoprotein of 55 kD. Minor but significant amounts of the class I histocompatibility antigen Qa-2 are also contained in the detergent-resistant complex. The membrane-associated proteins are not vectorially labelled, and form a complex group of proteins in the 30-70 kD range. Since actin is not detectable among these polypeptides, they probably constitute a plasma membrane-associated structure that is distinct from actin-containing, submembranous cytoskeletal elements.