Allolactose is a disaccharide similar to lactose.

It may arise from the occasional transglycosylation of lactose by β-galactosidase.

It is an inducer of the lac operon in Escherichia coli and many other enteric bacteria.

The absence of the repressor allows the transcription of the lac operon to proceed.

A non-hydrolyzable analog of allolactose, isopropyl β-D-1-thiogalactopyranoside (IPTG), is normally used in molecular biology to induce the lac operon.