To study the role of organic matter in ice crystal formation, we investigated the ice nucleation ability of a subcomponent of organic aerosols, the biopolymer lignin, using a droplet-freezing technique. We found that lignin is an ice-active macromolecule with changing abilities based on dilutions. The effects of atmospheric processing and of physicochemical treatments on the ability of lignin solutions to freeze were negligible. Thus, lignin is a recalcitrant ice-nucleating macromolecule.