The rocket spectra taken by Morton et al. (1967) were used as a basis for the investigation. These are in the far ultraviolet region of the Orion supergiants δ, ∊, ζ Orionis. Because of the similarity in spectral type and luminosity of these stars and the great strength of the shifted resonance lines it was possible to derive a mean P Cygni profile (standard error 5 % of the continuum) representing a typical single ultraviolet resonance line for a star of spectral type ~B0, Mv ~ −6.2. This is the ‘observed’ mean profile in Figures 2 and 3.