Over the last 20 years, the success of Japanese manufacturing firms has brought renewed attention to the importance of cost reduction on existing products as a source of productivity growth. This paper uses survey data and field interviews from the auto supply industry to explore the determinants of average-cost reduction for a sample of 171 plants in the United States and Canada between 1988 and 1992. The main result is that the determinants of cost reduction differ markedly between firms which had employee involvement programs in 1988 and firms that did not. The two groups of firms achieved equal amounts of cost reduction. but did so in very different ways. Firms with employee involvement saw their costs fall more if they also had "voice" relationships with customers and workers. Firms without such involvement gained no cost-reduction benefit fkom these programs; instead, their cost reduction success was largely a fiction of increases in volume. These results provide support for Milgrom and Roberts's concept that certain production practices exhibit complementary.