We consider batch processing of jobs to minimize the mean response time. A batch processing machine can handle up to B jobs simultaneously. Each job is represented by an arrival time and a processing time. The processing time of a batch of several jobs is the maximum of their processing times. In batch processing, non-preemptive scheduling is usually required and we discuss this case. The batch processing problem reduces to the ordinary uni-processor system scheduling problem if B = 1. We focus on the case B = +∞. Even for this seemingly simple extreme case, we are able to show that the problem is NP-hard. We further show that several important special cases of the problem can be solved in polynomial time.