1. [adj]  (of a specimen for study under a microscope) not treated with a reagent or dye
2. [adj]  not subjected to chemical or physical treatment; "an untreated fabric"
3. [adj]  not given medical care or treatment; "an untreated disease"; "the untreated wounded lay on makeshift cots"

UNTREATED is a 9 letter word that starts with U.