Are car assembly plants which have adopted 'lean production' trading with 'lean' parts suppliers? Or are they using suppliers as buffers and as sources of low cost labour? This paper examines these questions empirically by using official statistics in the UK, US, Germany and Japan. In the UK, there is evidence of parallel reduction in inventories at suppliers and assemblers in the 1980s, but also of growing productivity and wage gaps between the two groups. It is argued that management, unions and policy-makers should all be concerned about this emergent dualism in the UK automotive industry, and that attention should be paid to the mechanisms for diffusing innovative practices from assemblers to suppliers.