- O-Nitrophenyl-β-D-galactopyranoside (ONPG) is structurally a modified lactose that provides a colour indicator of lactose fermentation
- Tests for the presence of β-galactosidase
- Positive if the media turns yellow; usually positive within an hour, but cannot be called negative until 24 hours
- When positive, identifies the organism as a lactose fermenter, and may be quicker for "late" fermenters than other methods
