We calcuate the ion density observable by the plasma instrument on Pioneer 10 assuming that all ions are injected by Io within the radial range 5.8<L<6.2 and are transported away by radial diffusion by flux tube interchange motion. To account for the observations of Frank et al. (1976), one can postulate an injection spectrum which contains a 400-eV Maxwellian, a 5-keV almost monoenergetic distribution, and a 13-keV almost monoenergetic distribution. The 400-eV contribution could be sodium or other heavy ions. The other two contributions would have to be due to some kind of acceleration mechanism. Io-injected particles must have undergone pitch angle scattering in order to be detectable by the plasma instrument. An alternative explanation of the observations is that there is only injection by Io of the 5-keV ions (or lower energy ions which are rapidly lost in the vicinity of Io), whereas the plasma inside and outside the Io L shell is due to an ionospheric source.