Measurements of the vertical flux of O+ in the F region have been made by the incoherent scatter technique at Millstone Hill. In part 2 of this paper, evidence was presented for the existence of fluxes of ≥ 108 cm−2 sec−1 into the protonosphere during the day. In this paper we employ the altitude variation of the O+ flux to infer the magnitude of the proton flux in the charge-transition region. Based on continuity considerations and the theory of minor ion diffusion in the topside of the F layer, we have attempted to determine the abundance of H+ in the altitude interval where it is a minor ion. The results indicate that the abundance of neutral hydrogen near 600 km must exceed about 105 cm−3. However, the absence of detectable amounts of H+ in the signal spectra taken near 1000 km indicates that the transition altitude must be above 1000 km and in turn the neutral-hydrogen abundance cannot exceed this estimate by more than a factor of 3.