We describe the trigger system of the ALICE dimuon spectrometer, based on RPC detectors. The background rate is at least one order of magnitude less in ALICE than in LHC p-p experiments due to the lower luminosity in ion running. Hence we study the possibility to operate the RPCs either in streamer or proportional mode. We report a study done with cosmic rays concerning gas mixtures for streamer mode. The aim is to minimise the streamer charge for a better flux capability of the RPC and a smaller cluster size, as needed for ALICE. We also give the results of a beam test at SPS where the RPC was placed in the vicinity of a hadron absorber, simulating the operation conditions in ALICE
