This paper estimates the effect of union work rules in the building trades on employment and costs by comparing factor demand elasticities for union and nonunion contractors and subcontractors over micro data from two different types of construction. The results show that the elasticities of substitution between labor and nonlabor inputs and own-price elasticities for nonlabor inputs are about the same for union and nonunion contractors. In contrast, the elasticities of substitution among different skill categories of labor and the own-price elasticities for each category are much lower under unionism.