Two lines of research on cross-task priming yield opposite results. Research on repetition priming observed positive priming, whereas research on the role of priming in task-switching observed negative effects. We combined the two types of design. In the transfer phase of our paradigm, subjects performed task B either as a pure block (BBB) or as a switch block (ABAB). We presented items which were either unprimed or primed by prior presentation during a preceding priming phase performed on task A. Amongst others, the priming effect is determined by two factors: First, the more operation time the system needs during the probe event, the higher the likelihood to obtain priming. Protracting operation time by reducing stimulus quality favors positive priming, whereas providing more operation time by making subjects switch between tasks favors negative priming. Second, the strength of the memory trace of the prime event determines whether that trace can possibly yield negative priming, in that only strong traces can be retrieved together with the associated task/response.