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InUS law, the termillegalper se means that the act is inherently illegal. Thus, an act is illegal without extrinsic proof of any surrounding circumstances such as lack ofscienter (knowledge) or other defenses. Acts are made illegalper se bystatute,constitution orcase law.

Manydrunk driving laws make driving with ablood alcohol content over a certain limit (such as 0.05% or 0.08%) an act which is illegalper se.

In theUnited States, illegalper se often refers to categories ofanti-competitive behavior inantitrustlaw conclusively presumed to be an "unreasonable restraint on trade" and thus anti competitive. TheUnited States Supreme Court has, in the past, determined activities such asprice fixing, geographicmarket division, andgroup boycott to be illegalper se regardless of the reasonableness of such actions. Traditionally, illegalper se anti-trust acts describehorizontal market arrangements amongcompetitors.

The illegalper se category can trace its origins in the 1898 Supreme Court caseAddyston Pipe & Steel Co. v. U.S.,175 U.S. 211 (1898).

A number of cases have subsequently raised doubts about the validity of the illegalper se rule. Under modern Antitrust theories, the traditionally illegalper se categories create more of apresumption of unreasonableness.^[1] The court carefully narrowed theper se treatment and began issuing guidelines. Courts and agencies seeking to apply theper se rule must:

1. show "the practice facially appears to be one that would always or almost always tend to restrict competition and decrease output";
2. show that the practice is not "one designed to 'increase economic efficiency and render markets more, rather than less, competitive'";
3. carefully examine market conditions; and
4. absent good evidence of competitiveness behavior, avoid broadeningper se treatment to new or innovative business relationships.

• Malum in se
• Negligenceper se
• Presumption
• Rule of reason
• Strict liability (criminal)

• Competition law
• American legal terminology

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