Currently available on‐line systems historically have been used as retrospective search methodologies. With interest in on‐line retrieval increasing by leaps and bounds, the application of these inverted‐file oriented systems to selective dissemination of information (SDI) services bears investigation. Within given user‐specified demands and system constraints, what is the best method of providing an SDI service? A typical batch system is compared with four types of commonly available on‐line SDI systems. The operating parameters of each are outlined, and estimates of bottom‐line costs are given.