Because she didn't think it would wash until the studio Luxed it and it came out lovely. Was she impressed!
8 seconds sound clip from the Notorious (1948, Lux Radio Theatre) classic radio play.
You can hear this line at 00:37:14 in the radio play.
Quote context
[...]
- But I did have a twinge of envy when I saw those gorgeous clothes designed for Merle Oberon.
- Merle told me she was going to have it copied for her own wardrobe.
- And, er, John Kennedy might be interested in something else she said about it.
- Oh, yes.
- R.K.O., just like other leading Hollywood studios insists on Lux care for all nice washables.
- So, what could be safer for a star's own lovely things? Or for any girl's?
- Well, that's for me, Mr. Kennedy. But how do you figure it?
- Well, underthings washed the Lux way stay lovely three times as long. Tests prove that.
[...]