Problems in abundance determination from UV spectra of hot supergiants

We present measurements of equivalent widths of the UV, presumably photospheric lines: C III 1247 A, N III 1748 A, N III 1752 A, N IV 1718 A and He II 1640 A in high-resolution IUE spectra of 24 galactic OB supergiants. Equivalent widths measured from the observed spectra have been compared with their counterparts in the Tlusty NLTE synthetic spectra. We discuss possibilities of static plan-parallel model to reproduce observed UV spectra of hot massive stars and possible reasons why observations differ from the model so much.