THE GTP-binding proteins are well-known regulators of cellular functions(1-4), including vesicular transport(5-7). Cholera toxin, which is known to catalyse ADP-ribosylation of the as subunit of heterotrimeric G proteins, stimulates secretory vesicle formation from the trans-Golgi network(8). Here we describe a new cholera toxin target, an ’extra large’ G protein (XLas; M(r) 92K) which consists of a new 51K XL-portion linked to a Gas truncated at the amino terminus. XLas is specifically associated with the trans-Golgi network and occurs selectively in cells containing both the regulated and the constitutive pathway of protein secretion. Hence, XLas may mediate the effects of cholera toxin on secretory vesicle formation.