The unidentified stellar emission lines at 4485 A and 4503 A have been detected in O mainsequence spectra by means of high-resolution, high-S/N observations. Previously these lines were known only in mid-to late-O giant and supergiant spectra; the present result indicates that they have a positive luminosity dependence, similar to that of other selective emission features in O-type spectra. The lines remain unidentified, although there is morphological evidence that they are not due to CNO

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... suggesting an origin from some heavier species. These same observations provide high-quality data on the little-studied interstellar diffuse band at 4502 A, the redward wing of which may overlap the stellar X4503 emission. In lower-resolution observations of the luminous central clusters in the distant giant H II regions NGC 3603 and RCW 49, the X4502 absorption intensity is similar to that of the stellar helium lines, while interstellar X4430 is comparable with the hydrogen lines.