The objective of operations on futures markets may be either hedging or speculation. In this paper, we wish to give a desciption of futures markets with two groups of operators with heterogeneous expectations: hedgers-speculators, and pure speculators. The existence of carry-over costs is taken into account in the case of commodity trading, as well as in the case of financial futures. An equation, giving a simplified expression of the futures price, is derived. Applications to nonferrous metal futures (aluminium, copper and nickel) commodity markets are proposed.