At least three different glucose transport systems exist in mammalian cells. These are: (1) the constitutively active, facilitative carrier characteristic of human erythrocytes¹, Hep G2 (ref. 2) cells and rat brain³; (2) the Na-dependent active transporter of kidney and small intestine^4,5; and (3) the facilitative carrier of rat liver (B. Thorens and H. F. Lodish, personal communication). A fourth possible glucose transport system is the insulin-dependent carrier that may be specific to muscle and adipose tissue⁶. This transporter resides primarily in an intracellular compartment in resting cells from where it translocates to the cell surface upon cellular insulin exposure^7–9. This raises the question of whether hormonal regulation of glucose transport is conferred by virtue of a tissue-specific signalling mechanism or a tissue-specific glucose transporter. Here we present data supporting the latter concept based upon a monoclonal antibody against the fat cell glucose transporter that identifies a unique, insulin-regulatable glucose transport protein in muscle and adipose tissue.