We present very accurate individual masses (1.2% relative accuracy) for the two components of Gl 570BC, an interferometric and double-lined spectroscopic binary system. They were obtained from new high accuracy radial velocity and angular separation measurements, analysed together with previously published measurements. From those data we determine a much improved orbit through a simultaneous least square fit to the radial velocity, visual, and parallax information. The derived masses and absolute magnitudes generally validate the theoretical and empirical mass-luminosity relations around 0.5 M_sun, but point towards some low level discrepancies at the 0.1 to 0.15 magnitude level. Forthcoming results of this observing program will extend the comparison to much lower masses with similar accuracy. Partly based on observations made at Observatoire de Haute-Provence, operated by the Centre National de la Recherche Scientifique de France, on observations made at Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii, at the 3.6m telescope of the European Southern Observatory at La Silla (Chile), and the Kitt Peak Mayall 4m telescope operated by NOAO. Table 2 is only available electronically with the On-Line publication at http://linl.springer.de/link/service/journals/00230/