In the case of American Medical Association v. United States, 1942, the U.S Supreme Court ruled that a group of doctors had violated antitrust laws by conspiring to prevent a nonprofit health plan from operating in D.C. The doctors argued that their profession should be exempt from such laws because they provide a service rather than trade commodities. However, the court rejected this argument stating that any form of commerce - whether it's trading goods or providing services - falls under federal regulation if it affects more states than one. This decision was significant as it established precedent for future cases involving medical professionals and antitrust law enforcement.

In the dissenting opinion for the American Medical Association v. United States case, Justice Roberts argued that the majority's decision was a misinterpretation of both antitrust laws and previous court decisions. He contended that professional organizations like AMA should not be considered as engaging in trade or commerce under Sherman Act, thus their activities cannot constitute restraint of trade. He also pointed out that doctors were not selling goods but providing services which he believed to fall outside the scope of 'trade' defined by law. Furthermore, he disagreed with majority’s view on group boycotts being per se illegal; instead, he suggested each case should be evaluated individually considering its circumstances and effects on competition rather than applying blanket rules. Lastly, Justice Roberts expressed concerns about potential negative impacts this ruling could have on other professional associations whose actions might now be scrutinized under antitrust laws despite their intentions to uphold standards within their professions.