It is primarily traditional fining agents that make a wine unsuitable for vegans.  Some winemakers still use traditional animal-derived fining agents such as egg whites, gelatin, or casein (derived from milk) to clarify the liquid, reduce bitterness, or bind and extract excess tannins in red wines, leaving behind softer ones. The agents are removed before bottling, but the wine cannot be considered vegan.

Vegan wines are made without animal products, so winemakers either allow the particles to sink naturally to the bottom of the wine, or use non-animal fining products, usually bentonite, a form of clay, or pea protein to clarify the liquid.  Just because a wine is organic, biodynamic, natural, or kosher does not mean it is also vegan.

Other animal products used in wine production may include beeswax (used to seal bottles) and agglomerated corks (which use milk-based glues).  To be considered vegan, a wine must avoid using these products as well.