This paper addressed the problem of the optimization of the input order of lots in a multiproduct batch processing system. This type of plant configuration is typically used in the production of paints, medicines, cosmetics, electronics components, etc. The dimension of this scheduling problem is, in a general way, very high. An optimization technique called Simulated Annealing (SA), successfully used to solve several large combinatorial problems (for example, traveling salesman problem), was implemented for determining the input order of a given set of products so as to minimize the average residence time of these products in the plant with Unlimited Intermediate Storage (UIS) policy. After some medium size trials, the proposed technique is applied to a real world problem - a semiconductor circuit fabrication plant of MOTOROLA Inc. (Toulouse).