Observations of large-amplitude MHD waves upstream of the Jovian bow shock have previously been interpreted as arising from a resonant electromagnetic ion beam instability (Goldstein et al., 1983). That interpretation was based on the conclusion that the observed fluctuations were predominantly right elliptically polarized in the solar wind rest frame. Because it has been noted by the authors that the fluctuations are, in fact, left elliptically polarized (Smith et al., 1983; Goldstein et al., 1983), a reanalysis of the observations is necessary. In this paper we investigate several mechanisms for producing left-hand-polarized MHD waves in the observed frequency range. Instabilities excited by protons appear unlikely to account for the observations. We conclude that a resonant instability excited by relativistic electrons streaming along the magnetic field away from Jupiter is a likely source of free energy consistent with the observations. Evidence for the existence of such a population of electrons has been found in both the low energy charged particle experiments and cosmic ray experiments on Voyager 2. This new interpretation is reminiscent of observations reported by Smith et al. (1976) using Pioneer data of left-polarized (solar wind frame) MHD fluctuations in association with relativistic electrons.