Absorption spectra of interstellar grains. I - Reference stars In order to find the proper standards for the precise extraction ofinterstellar spectral features, an analysis was made of low-resolutionUV spectra of unreddened early-type stars. Stars having a E(B-V) colorexcess below 0.05m contained in the catalog of TD-1 satelliteobservations made during the S2/68 experiment are examined.