The principal Minerals Management Service (MMS) policy or regulation intended to promote competition (or inhibit collusion) in the lease market is the Restricted Joint Bidders List. It is compiled and published twice a year and applies to all lease sales held during the subsequent six-month period or until the next list is issued. This paper applies descriptive and econometric analyses to data on lease sales in the US Gulf of Mexico OCS region to evaluate the effectiveness of this policy. In the aggregate, empirical analyses suggest that the imposition of joint bid restrictions on some E&P firms reduces bidding effectiveness for petroleum leases on the OCS. The patterns of bidding for leases on the OCS by E&P firms restricted from bidding jointly do not seem to indicate anticompetitive behavior. Further, joint bidding is found to be consistently associated with higher average high bids and this seems to be consistent with, and perhaps enhances competition in the lease market. (c) 2006 Elsevier Ltd. All rights reserved.