High intensity H^- beams are injected into the K130 cyclotron for isotope production and for proton induced fission studies. Earlier, when protons were accelerated as positive ions, the beam intensity was limited by beam losses in the extraction system from the cyclotron. Stripping extraction of negative ions removed this limitation. However, now space charge effects in the injection beam line limit the beam intensity. At present, the maximum practical H^- beam intensity at the inflector is about 0.25 mA, which gives 40-50 mA of extracted proton beam. Calculations predict that the injection beam line from the ion source to the matching quadrupoles below the cyclotron can transfer about 1 mA of 6 keV H^- beam, which also was measured. The quadrupole section has a smaller transmission. Also a significant portion of the beam is lost during the last 2 m in the axial hole. General rules for maximum beam intensity as a function of beam line parameters such as beam tube aperture, distance of focusing elements, beam charge, mass and energy are given for different kinds of focusing systems (solenoids, FODO and FOFDOD quadrupole structures). As a conclusion, some suggestions to improve the transmission of the injection line are given.