The spectra of 34 galaxies within 20 arcmins (\sim 100 kpc) of the lenticular galaxy NGC 5084 have been obtained using the FOCAP system on the Anglo-Australian 3.9m telescope. Nine objects are found with projected separations \lesssim 80 kpc and with radial velocities within \pm 630 km/s of the parent galaxy redshift. Using various techniques, their velocity differences and projected separations are used to estimate the mass of this S0 galaxy, which ranges from 6 \times 10^{12} M_{\odot} to 1 \times 10^{13} M_{\odot}. With such a mass, NGC 5084 is one of the most massive disk galaxy known, with a M/L_B \gtrsim 200 M_{\odot}/L_{\odot}. In agreement with the models' predictions of Quinn & Goodman (1986) but contrary to the results of Zaritsky et al (1993) obtained from their statistical sample, the properties of the satellites' population show no evidence for the "Holmberg effect", and a clear excess of satellites in retrograde orbits. Several signs hint that this S0 galaxy has survived the accretion of several satellites.

20 pages Latex (aasms4.sty) including all tables and 4 postscript figures. AJ in Press