Douglas McGregor's landmark book, The Human Side of Enterprise, changed the path of management thinking and practice by questioning fundamental assumptions about human behavior in organizations. Although nearly 50 years have passed since McGregor's initial formulation of Theory X and Theory Y, the substantive validity of his theorizing on the effects of individual differences in managerial assumptions has remained virtually unexamined. In our view, the paucity of substantive research is attributable to the absence of a published measure of Theory X/Y managerial assumptions with adequate construct validity. This article describes the development and validation of such a measure. The pattern of correlations between the focal measure and conceptually identical, closely related, distally related, and unrelated measures (rs = .66, .51, .25, and -.01, respectively) provides evidence of convergent, substantive, and discriminant validity. We see the present research as conceptually contributory in that we distinguish between practices consonant with Theory Y and managerial attitudes per se. Our work is empirically contributory by providing a construct-valid measure of Theory X/Y managerial assumptions, an instrument that permits the conduct of substantive research. Finally, if McGregor's theorizing is correct— and that remains to be determined— there are significant potential practical implications for the management of human behavior in organizations.