Recent work suggests that the labor elasticity of supply to the firm is finite. In other words, labor markets are not perfectly competitive. We provide the first estimates of the degree of labor market imperfections over a long period of time with significant variation in the state of the macroeconomy. Using data from the Ford Motor Company from 1918 through 1940 we analyze the labor elasticity of supply to the firm over the business cycle. We find significant variation in this parameter over time and consistent with predictions from a simple search model we find evidence of it being procyclical. Our analysis also contributes two methodological extensions to the empirical monopsony literature. First, we are able to relax the assumption that all employee separations in the firm are replaced by recruits. Second, we use data that allows us to isolate workers who quit for voluntary reasons. Both have significant impacts on our estimates.