During the years 1994-1997, the emulsion target of the CHORUS detector was exposed to the CERN-SPS Wide Band Neutrino Beam. The improvements of the past few years in the automatic emulsion scanning systems allowed a sample of events located in emulsion to be used for studies of charm production. Based on a sample of 56 172nu(mu) charged-current interactions analysed so far, we find a value of B-mu = 0.093+/-0.009(stat.) +/-0.009(syst.) for the semi-leptonic branching fraction of charm hadrons. The result for events with visible energy larger than 30 GeV, B-mu = 0.102+/-0.016(stat.) +/-0.010(syst.), can be combined with the existing measurements of the dimuon rate to yield a value of 0.219+/-0.022 for the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element \V-cd\. (C) 2002 Elsevier Science B.V. All rights reserved.